What can be said about the function of testosterone?

Testosterone plays a crucial role in the development and maintenance of male sexual characteristics. It is the primary male sex hormone and is responsible for the physical changes that occur during puberty, such as the deepening of the voice, growth of facial and body hair, and increased muscle mass. Additionally, testosterone is essential for the development of the testes and prostate, as well as influencing sex drive and fertility. This hormone also has effects on various organs and tissues throughout the body, but its most significant role is centered on male sexual differentiation and characteristics.

The other choices, while related to various physiological processes, do not accurately describe the primary function of testosterone. For example, regulating the basic metabolic rate pertains more to thyroid hormones, increasing breast milk production is primarily influenced by prolactin, and inhibiting blood glucose production is more related to insulin and glucagon's function.