Which example reflects ad hominem logical fallacy?

A.
Students should protest against the principal’s policy on prom parties because he's too old to understand what teenagers want.
B.
Spending time studying in the library every day after school is better than never cracking a book.
C.
Investing in the stock market is like gambling in a casino. There’s no guarantee that you’ll profit.
D.
Yes, the governor has made some mistakes during his term. Former governors made mistakes too. Making mistakes is only human, and it is a good practice to forgive people.