Explanation
The statement that can be used to expose the field "connid" of the data source "demo_ods_assoc_spfli" in the element list is A. demo_ods_assoc_spfli.connid,.
This statement uses the dot notation to access the field "connid" of the data source "demo_ods_assoc_spfli", which is an association defined on line #4. The association "demo_ods_assoc_spfli" links the data source "demo_ods" with the table "spfli" using the field "carrid".
The statement also ends with a comma to separate it from the next element in the list12.
You cannot do any of the following:
B). demo_ods_assoc_spfli-connid/: This statement uses the wrong syntax to access the field "connid" of the data source "demo_ods_assoc_spfli". The dash notation is used to access the components of a structure or a table, not the fields of a data source. The statement also ends with a slash, which is not a valid separator for the element list12.
C). spfli-connid,: This statement uses the wrong data source name to access the field "connid". The data source name should be "demo_ods_assoc_spfli", not "spfli". The statement also uses the wrong syntax to access the field "connid", as explained above12.
D). _spfli.connid/: This statement uses the wrong data source name and the wrong separator to access the field "connid". The data source name should be "demo_ods_assoc_spfli", not "_spfli". The statement also ends with a slash, which is not a valid separator for the element list12.
References: 1: ABAP CDS - SELECT, select_list - ABAP Keyword Documentation - SAP Online Help 2: ABAP CDS - SELECT, from - ABAP Keyword Documentation - SAP Online Help

Explanation
The patterns that raise an exception are those that use the constructor operator EXACT to perform a lossless assignment or calculation, but the result cannot be converted to the target data type without data loss. The following are the explanations for each pattern:
A: This pattern raises the exception CX_SY_CONVERSION_LOST because the result of the calculation 2 * 3 is 6, which cannot be assigned to a packed number with two decimal places without losing the integer part. The operator -U is used to perform a lossless calculation with the calculation type decfloat34.
B: This pattern does not raise an exception because the result of the substring expression gco_string+5(5) is '6789A', which can be assigned to a string without data loss. The operator EXACT # is used to perform a lossless assignment with the data type of the argument.
C: This pattern raises the exception CX_SY_CONVERSION_LOST because the result of the substring expression gco_string+5(6) is '6789AB', which cannot be assigned to a character field with length 5 without losing the last character. The operator EXACT is used to perform a lossless assignment with the data type of the target field.
D: This pattern does not raise an exception because the result of the calculation 2 / 2 is 1, which can be assigned to a packed number with three decimal places without data loss. The operator -U is used to perform a lossless calculation with the calculation type decfloat34.
E: This pattern raises the exception CX_SY_CONVERSION_ERROR because the constant gco_date contains an invalid value '20331233' for a date data type, which cannot be converted to a valid date.
The operator EXACT is used to perform a lossless assignment with the data type of the target field.
References: EXACT - Lossless Operator - ABAP Keyword Documentation, Lossless Assignments - ABAP Keyword Documentation

Explanation
The code snippet in the image is an example of using the FOR statement to create an internal table with a constructor expression. The FOR statement introduces an iteration expression that runs over the content of source_itab and assigns each row to the variable row. The variable row is then used to populate the fields of target_itab12. Some of the correct statements about the code snippet are:
FOR defines a loop that runs over the content of source_itab: This is true. The FOR statement iterates over the rows of source_itab and assigns each row to the variable row. The iteration expression can also specify a range or a condition for the loop12.
row is only visible within the loop: This is true. The variable row is a local variable that is only visible within the scope of the iteration expression. It cannot be accessed outside the loop12.
You cannot do any of the following:
source_itab is only visible within the loop: This is false. The variable source_itab is not a local variable that is defined by the FOR statement. It is an existing internal table that is used as the data source for the iteration expression. It can be accessed outside the loop12.
row is a predefined name and cannot be chosen arbitrarily: This is false. The variable row is not a predefined name that is reserved by the FOR statement. It is a user-defined name that can be chosen arbitrarily. However, it must not conflict with any existing names in the program12.
References: 1: FOR - Iteration Expressions - ABAP Keyword Documentation - SAP Online Help 2: ABAP 7.4 Syntax - FOR Loop iteration | SAP Community

The response will be an activation error because the field names of the union do not match. This is because the field names of the union must match in order for the definition to be activated. The union operator combines the result sets of two or more queries into a single result set. The queries that are joined by the union operator must have the same number and type of fields, and the fields must have the same names1. In the given code, the field names of the union do not match, because the first query has the fields carrname, connid, cityfrom, and cityto, while the second query has the fields carrname, carrier_id, cityfrom, and cityto. The field connid in the first query does not match the field carrier_id in the second query. Therefore, the definition cannot be activated.
References: 1: UNION - ABAP Keyword Documentation