Which endocrine function is the testis primarily known for?

The testis is primarily known for its endocrine function of producing testosterone. Testosterone is a crucial hormone that plays a significant role in male reproductive development, influencing various physical characteristics such as muscle mass, fat distribution, and the growth of facial and body hair. It also contributes to the production of sperm, testosterone synthesis is vital for supporting libido and sexual function.

In contrast, the other options refer to functions that are not primarily carried out by the testis. For example, waste excretion is mainly managed by the kidneys; water balance regulation is primarily a function of the kidneys and glands such as the pituitary; and the development of the immune system is largely influenced by the thymus and bone marrow. Thus, the key role of the testis in hormone production distinctly identifies its primary endocrine function as the production of testosterone.