Which hormone is responsible for stimulating the functions of the testes and ovaries?

Gonadotropin is the hormone responsible for stimulating the functions of the testes and ovaries. This hormone is made up of two major types: Luteinizing hormone (LH) and Follicle-stimulating hormone (FSH), both of which play key roles in regulating reproductive processes. In males, LH stimulates testosterone production by the testes, while FSH is crucial for spermatogenesis. In females, LH triggers ovulation and the formation of the corpus luteum, whereas FSH stimulates the growth of ovarian follicles.

The other options are not primarily responsible for the stimulation of the testes and ovaries. Estrogen is mainly involved in regulating female reproductive functions and is produced in the ovaries. Testosterone is the primary male sex hormone that is also produced in the testes but is not responsible for stimulating the functions of both gonads. Insulin is a hormone that regulates glucose metabolism and has no direct role in regulating the functions of the testes and ovaries. Thus, gonadotropins are the correct choice given their direct involvement in stimulating and regulating reproductive functions across both genders.