If the employee taking FMLA leave has unused paid vacation or sick leave, the employer may:

A. require the employee to use sick leave toward the FMLA leave, but not vacation.
B. not allow any time to be taken under the FMLA.
C. require the employee to use both sick leave and vacation toward the FMLA leave.
D. require the employee to use vacation toward the FMLA leave, but not sick leave.

is it C ?

Correct.