What is the primary effect of testosterone in the male body?

Testosterone plays a crucial role in the male body primarily by promoting the development of male reproductive tissues and secondary sexual characteristics. This includes the growth of the testes and prostate, increased muscle mass and strength, the development of facial and body hair, and the deepening of the voice during puberty. Furthermore, testosterone influences libido and contributes to various metabolic processes.

While some of the other options mention important metabolic functions or effects that might be associated with hormones, they do not capture the fundamental role of testosterone, which is essential for male sexual differentiation and the maintenance of male-specific traits and functions throughout life.