Bill's facial hair is an example of a (n)

A) secondary sex characteristic.

B) primary sex characteristic.

C) gender conformity behavior.

D) within-group variation.

I assume here that we mean the thick, characteristic hair that does not appear in women (since women also have some thin and white facial hair) - this is something that appears only at puberty and it's a physical characteristics.

This is then an example of a secondary sex characteristics - the answer is A).