Which statement is an example of an ad hominem attack?

A. The only way to improve America's educational system is to improve its students' test scores.
B. I come from a long line of cattle ranchers. I know what small-town living is all about.
C. If you don't donate to my cause, I'll know you don't really care about childhood poverty.
D. My opponent is an inefficient slob. Clearly, he has no idea how to manage his own life, much less this town.