What is the primary function of follicle-stimulating hormone (FSH) in both males and females?

Follicle-stimulating hormone (FSH) plays a crucial role in the regulation of reproductive processes in both males and females. In females, FSH is integral to the ovarian cycle, as it promotes the growth and maturation of ovarian follicles, which house the oocytes (eggs). This hormone is essential for the development of these follicles to the point where they can release a mature egg during ovulation.

In males, FSH is important for spermatogenesis, as it acts on the Sertoli cells in the testes to support the production and maturation of sperm. This hormone underscores the reproductive system's functioning by facilitating the development and maturation of gametes in both sexes.

The other options, although significant in different contexts, do not directly relate to the primary function of FSH. Muscle growth is primarily influenced by anabolic hormones like testosterone; water retention is regulated through hormones such as antidiuretic hormone; and metabolic processes are largely controlled by thyroid hormones and insulin. Thus, the primary function of FSH is indeed to regulate reproductive processes, making it the correct answer.