Which statement is true about the male reproductive system?
a. the testes produce semen.
b. sperm cells are the largest component in the composition of semen.
c. the scrotum functions to keep the testes at an optimum temperature.
d. the penis enables sperm to be deposited directly into the uterus.

C. The scrotum functions to keep the testes at an optimum temperature.

Explanation:

In order to understand the correct answer, we need to review basic anatomy and physiology of male reproductive system.

Testes also known as testicles are paired organs located in the scrotum (bag of skin and smooth muscle) suspended from the body. Testes are responsible for the formation of sperms. Scrotum keeps the testicles away from the body. It provides a temperature cooler than the body temperature which is necessary for formation of sperms (Spermatogenesis). Sperms mix with other secretions from various internal genital organs (prosate, Cowper's glands, seminal vesicles) to form semen. Sperms constitute a very small proportion of semen.

Semen is deposited by the penis into vaginal tract from where the sperms travel to uterus.

Coming back to the answer choices,

A. The testes produce semen.

Testes produce sperms and semen is formed after sperms mix with other secretions of organs of male reproductive system.

B. Sperm cells are the largest component in the composition of semen.

Sperms constitute a small proportion of semen. Other components of semen include fructose, fibrinogen, coagulants, cholesterol etc.

C. The scrotum functions to keep the testes at an optimum temperature.

This is the correct answer choice as scrotum keeps testicles away from the body at a suitable temperature for spermatogenesis.

D. The penis enables sperm to be deposited directly into the uterus.

Penis deposits semen into the vaginal tract from where sperms travel to the uterus.