For the other options, you have "Count of xxx", so these require you to have a summary-type report. Only a) does not have an aggregate so you can use a list-type report. Typically, a sub-report is of the list type.
As per your above statement, Count of cannot be sub report. But in the CSSA Quiz states like answer is "C" (Columns to display are: Operator Name, Count of Operator Divisions he belong to) is correct answer. Please clarify my doubt. Thanks in advance.