When using the GROUP BY clause, every column in the SELECT clause that is not an aggregate function must be included in the GROUP BY clause:
A . SELECT dept_id, join_date, SUM(salary) FROM employees GROUP BY dept_id, join_date: This is a valid query because all non-aggregate columns in the SELECT list (dept_id and join_date) are included in the GROUP BY clause.
Queries B and C are invalid because they attempt to nest aggregate functions, which is not allowed. Query D is invalid because join_date is not included in the GROUP BY clause.
Reference:
Oracle Database SQL Language Reference 12c, specifically the section on GROUP BY clause constraints.