In Oracle 12c SQL, the GROUP BY clause is used to arrange identical data into groups with the GROUP BY expression followed by the SELECT statement. The SUM() function is then used to calculate the sum for each grouped record on a specific column, which in this case is the salary column. Option A is valid because it correctly applies the GROUP BY clause. Both dept_id and join_date are included in the SELECT statement, which is a requirement when using these columns in conjunction with the GROUP BY clause. This means that the query will calculate the sum of salaries for each combination of dept_id and join_date. It adheres to the SQL rule that every item in the SELECT list must be either an aggregate function or appear in the GROUP BY clause. Option B is invalid due to a typo in SELECT depe_id and also because it ends with a colon rather than a semicolon. Option C is invalid because you cannot nest aggregate functions like MAX(AVG(salary)) without a subquery. Option D is invalid for the same reason as option C, where it tries to nest aggregate functions AVG(MAX(salary)), which is not allowed directly in SQL without a subquery. For further reference, you can consult the Oracle 12c documentation, which provides comprehensive guidelines on how to use the GROUP BY clause and aggregate functions like SUM(): Oracle Database SQL Language Reference, 12c Release 1 (12.1): GROUP BY Clause Oracle Database SQL Language Reference, 12c Release 1 (12.1): Aggregate Functions