is an example of a primary sex characteristic.
A.
The production of sperm
B.
The growth of body hair
C.
The development of breasts
D.
The deepening of a male's voice

to provide benchmarks

out of the choices i'd say d the ability to set goals, mainly because to desire to continue you need motivation but it isn't motivation in that part, but setting goals means you have an endgame and you want to reach it which sums up motivation.