The following are the explanations for each ABAP SQL statement:
A: This statement is valid. It selects the fields carrid, airpfrom, and the aggregate functions MAX(distance) and MIN(distance) from the table /dmo/connection, and groups the results by carrid and airpfrom. The aggregate functions are aliased as dist_max and dist_min. The results are stored in an internal table named It_hits, which is created using the inline declaration operator @DATA.
B: This statement is valid. It is similar to statement A, except that it does not specify the GROUP BY clause. This means that the aggregate functions are applied to the entire table, and the results are stored in an internal table named It_hits, which is created using the inline declaration operator @DATA.
C: This statement is invalid. It selects the aggregate functions MAX(distance) and MIN(distance) from the table /dmo/connection, but it does not specify any grouping or non-aggregate fields. This is not allowed in ABAP SQL, as the SELECT list must contain at least one non-aggregate field or a GROUP BY clause. The statement will cause a syntax error.
D: This statement is invalid. It selects the fields carrid and airpfrom from the table /dmo/connection, and groups the results by carrid and connid. However, the field connid is not included in the SELECT list, which is not allowed in ABAP SQL, as the GROUP BY clause must contain only fields that are also in the SELECT list. The statement will cause a syntax error.

String functions are expressions that can be used to manipulate character-like data in ABAP. String functions can be either predicate functions or non-predicate functions. Predicate functions are string functions that return a truth value (true or false) for a condition of the argument text. Non-predicate functions are string functions that return a character-like result for an operation on the argument text1.
The following string functions are predicate functions:
* B. contains_any_of(): This function returns true if the argument text contains at least one of the characters specified in the character set. For example, the following expression returns true, because the text 'ABAP' contains at least one of the characters 'A', 'B', or 'C':
contains_any_of( val = 'ABAP' set = 'ABC' ).
* D. matches(): This function returns true if the argument text matches the pattern specified in the regular expression. For example, the following expression returns true, because the text 'ABAP' matches the pattern that consists of four uppercase letters:
matches( val = 'ABAP' regex = '[A-Z]{4}' ).
The following string functions are not predicate functions, because they return a character-like result, not a truth value:
* A. find_any_not_of(): This function returns the position of the first character in the argument text that is not contained in the character set. If no such character is found, the function returns 0. For example, the following expression returns 3, because the third character of the text 'ABAP' is not contained in the character set 'ABC':
find_any_not_of( val = 'ABAP' set = 'ABC' ).
* C. count_any_of(): This function returns the number of characters in the argument text that are
* contained in the character set. For example, the following expression returns 2, because there are two characters in the text 'ABAP' that are contained in the character set 'ABC':
count_any_of( val = 'ABAP' set = 'ABC' ).
References: 1: String Functions - ABAP Keyword Documentation

To adhere to the most recent ABAP SQL syntax conventions from SAP, you must insert the "INTO TABLE @gt flights" clause on line #4 to complete the SQL statement. This is because the INTO or APPENDING clause should be specified immediately after the SELECT clause, according to the ABAP SQL syntax conventions1. The INTO or APPENDING clause defines the data object to which the results set of the SELECT statement is assigned. The data object can be an internal table, a work area, or an inline declaration. In this case, the data object is an internal table named gt_flights, which is created using the inline declaration operator @DATA. The inline declaration operator allows you to declare and create a data object in the same statement where it is used, without the need for a separate DATA statement2.
The other lines are not suitable for inserting the "INTO TABLE @gt flights" clause, as they would violate the ABAP SQL syntax conventions or cause syntax errors. These lines are:
#6: This line is not suitable for inserting the "INTO TABLE @gt flights" clause, as it would cause a syntax error. This is because the FROM clause must be specified before the INTO or APPENDING clause, according to the ABAP SQL syntax conventions1. The FROM clause defines the data sources from which the data is read, such as database tables, CDS view entities, or CDS DDIC-based views. In this case, the data source is the database table flights.
#8: This line is not suitable for inserting the "INTO TABLE @gt flights" clause, as it would cause a syntax error. This is because the ORDER BY clause must be specified after the INTO or APPENDING clause, according to the ABAP SQL syntax conventions1. The ORDER BY clause defines the sort order of the results set of the SELECT statement. In this case, the results set is sorted by the fields carrid, connid, and fltime.
#15: This line is not suitable for inserting the "INTO TABLE @gt flights" clause, as it would violate the ABAP SQL syntax conventions. This is because the INTO or APPENDING clause should be specified as close as possible to the SELECT clause, according to the ABAP SQL syntax conventions1. The INTO or APPENDING clause should not be separated from the SELECT clause by other clauses, such as the WHERE clause, the GROUP BY clause, the HAVING clause, the UNION clause, or the ORDER BY clause. This is to improve the readability and maintainability of the ABAP SQL statement.

Explanation
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