Answer:
D. allows you to add calculations
Explanation:
GROUP ON is a database function that allows you to group together a series of results sharing common values for the indicated fields. That allows you to add calculations.
You could for example, count how many students are in Mrs Brown's class, or how many of them had a grade of 80% or more on the last test. Or instantly calculate the class' average for every teacher in the school, maybe grouped by teacher/level, and so on.
