Here are the step-by-step workings:
Optimistic time (Best case): 25 weeks
Most likely time: 27 weeks
Pessimistic time (Worst case): 32 weeks
PERT formula: (Optimistic + 4 * Most likely + Pessimistic) / 6
= (25 + 4 * 27 + 32) / 6
= (25 + 108 + 32) / 6
= 165 / 6
= 27.5 weeks
The answer is C.