Explanation
In class ZCL_CLASS_A, you use the statement DATA var TYPE *** to declare a data object named var with a data type specified by ***. The data type can be any of the following1:
A predefined ABAP type, such as i, f, c, string, xstring, and so on.
A data element from the ABAP Dictionary, such as matnr, carrid, bukrs, and so on. A data element defines the semantic and technical attributes of a data field, such as the domain, the length, the data type, the description, and the value range2.
A domain from the ABAP Dictionary, such as matnr_d, carrid_d, bukrs_d, and so on. A domain defines the technical attributes of a data field, such as the data type, the length, the output length, the number of decimal places, and the value range3.
A type defined globally in a class, an interface, or a type pool, such as zcl_class_b=>type_a, zif_interface_c=>type_b, ztype_pool_d=>type_c, and so on. A global type is a type that is defined in a global repository object and can be used in any program or class4.
A type defined locally in the current class, such as type_a, type_b, type_c, and so on. A local type is a type that is defined in the declaration part of a class and can only be used within the class5.
Therefore, the possible values for *** are B. the name of a data element from the ABAP Dictionary and D. the name of a domain from the ABAP Dictionary. The other options are not valid because:
A). The name of a type defined privately in class ZCL_CLASS_A is a local type and cannot be used with the DATA statement. A local type can only be used with the TYPES statement5.
C). The name of a type defined privately in another class is a private type and cannot be accessed from outside the class. A private type can only be used within the class that defines it.
References: 1: DATA - ABAP Keyword Documentation 2: Data Elements - ABAP Dictionary - SAP Online Help 3: Domains - ABAP Dictionary - SAP Online Help 4: Global Types - ABAP Keyword Documentation 5:
Local Types - ABAP Keyword Documentation : Private Types - ABAP Keyword Documentation

Explanation
The purpose of a foreign key relationship between two tables in the ABAP Dictionary is to ensure the integrity of data in the corresponding database tables. A foreign key relationship defines a logical link between a foreign key table and a check table, where the foreign key fields of the former are assigned to the primary key fields of the latter. This means that the values entered in the foreign key fields must exist in the check table, otherwise the system will reject the entry. This way, the foreign key relationship prevents the insertion of invalid or inconsistent data in the database tables.
A foreign key relationship also serves to document the relationship between the two tables in the ABAP Dictionary, but this is not its primary purpose. A foreign key relationship does not necessarily create a corresponding foreign key relationship in the database, as this depends on the database system and the settings of the ABAP Dictionary. Some database systems do not support foreign keys at all, while others require additional steps to activate them. Therefore, the foreign key relationship in the ABAP Dictionary is mainly a logical concept that is enforced by the ABAP runtime environment.
References: Foreign Keys (SAP Library - ABAP Dictionary), Foreign Keys (SAP Library - BC - ABAP Dictionary)
https://help.sap.com/doc/saphelp_snc70/7.0/en-US/cf/21ea77446011d189700000e8322d00/content.htm

Explanation
The following are the explanations for each statement:
A: This statement is valid. go_ifl may call method ml with go_ifl->ml(). This is because go_ifl is a data object of type REF TO ifl, which is a reference to the interface ifl. The interface ifl defines a method ml, which can be called using the reference variable go_ifl. The class cll implements the interface ifl, which means that it provides an implementation of the method ml. The data object go_ifl is assigned to a new instance of the class cll using the NEW operator and the inline declaration operator @DATA. Therefore, when go_ifl->ml() is called, the implementation of the method ml in the class cll is executed123 B: This statement is valid. Instead of go_cll = NEW #(...) you could use go_ifl = NEW cll(...). This is because go_ifl is a data object of type REF TO ifl, which is a reference to the interface ifl. The class cll implements the interface ifl, which means that it is compatible with the interface ifl. Therefore, go_ifl can be assigned to a new instance of the class cll using the NEW operator and the class name cll. The inline declaration operator @DATA is optional in this case, as go_ifl is already declared. The parentheses after the class name cll can be used to pass parameters to the constructor of the class cll, if any123 E: This statement is valid. go_ifl may call method m2 with go_ifl->m2(...). This is because go_ifl is a data object of type REF TO ifl, which is a reference to the interface ifl. The class cll implements the interface ifl, which means that it inherits all the components of the interface ifl. The class cll also defines a method m2, which is a public method of the class cll. Therefore, go_ifl can call the method m2 using the reference variable go_ifl. The method m2 is not defined in the interface ifl, but it is accessible through the interface ifl, as the interface ifl is implemented by the class cll. The parentheses after the method name m2 can be used to pass parameters to the method m2, if any123 The other statements are not valid, as they have syntax errors or logical errors. These statements are:
C: This statement is not valid. go_cll may call method ml with go_cll->ifl~ml(). This is because go_cll is a data object of type REF TO cll, which is a reference to the class cll. The class cll implements the interface ifl, which means that it inherits all the components of the interface ifl. The interface ifl defines a method ml, which can be called using the reference variable go_cll. However, the syntax for calling an interface method using a class reference is go_cll->ml(), not go_cll->ifl~ml(). The interface component selector ~ is only used when calling an interface method using an interface reference, such as go_ifl->ifl~ml(). Using the interface component selector ~ with a class reference will cause a syntax error123 D: This statement is not valid. Instead of go_cll = NEW #() you could use go_ifl = NEW #(...). This is because go_ifl is a data object of type REF TO ifl, which is a reference to the interface ifl. The interface ifl cannot be instantiated, as it does not have an implementation. Therefore, go_ifl cannot be assigned to a new instance of the interface ifl using the NEW operator and the inline declaration operator @DATA.
This will cause a syntax error or a runtime error. To instantiate an interface, you need to use a class that implements the interface, such as the class cll123 References: INTERFACES - ABAP Keyword Documentation, CLASS - ABAP Keyword Documentation, NEW - ABAP Keyword Documentation

Explanation
The data declarations that will always work without truncation or rounding for the assignment gv_target = gv_source are B and C. This is because the target data type string is a variable-length character type that can hold any character string, including those of data types c (fixed-length character) and d (date). The assignment of a character or date value to a string variable will not cause any loss of information or precision, as the string variable will adjust its length to match the source value12.
You cannot do any of the following:
A: DATA gv_source TYPE string, to DATA gv_target TYPE c.: This data declaration may cause truncation for the assignment gv_target = gv_source. This is because the target data type c is a fixed-length character type that has a predefined length. If the source value of type string is longer than the target length of type c, the source value will be truncated on the right to fit the target length12.
D: DATA gv_source TYPE p LENGTH 8 DECIMALS 3. to DATA gv_target TYPE p LENGTH 16 DECIMALS 2.: This data declaration may cause rounding for the assignment gv_target = gv_source.
This is because the target data type p is a packed decimal type that has a predefined length and number of decimal places. If the source value of type p has more decimal places than the target type p, the source value will be rounded to the target number of decimal places12.
References: 1: ABAP Data Types - ABAP Keyword Documentation - SAP Online Help 2: ABAP Assignment Rules - ABAP Keyword Documentation - SAP Online Help