Which of the following is influenced by testosterone?

Testosterone is a key hormone in male physiology and plays a crucial role in various biological processes. The development of sperm cells is directly stimulated by testosterone, as it is essential for spermatogenesis, the process of sperm production in the testes. Furthermore, testosterone is responsible for the development of secondary male sex characteristics, which include features such as increased muscle mass, body hair growth, and a deeper voice. This makes the option regarding the development of sperm cells and secondary male sex characteristics the correct choice, as it accurately reflects testosterone's significant effects on male reproductive and secondary sexual development.

In contrast, the growth of breast tissue is primarily influenced by hormones such as estrogen rather than testosterone. The regulation of the menstrual cycle involves various hormones, particularly estrogen and progesterone, and is not directly influenced by testosterone. Lastly, fat distribution in females is also largely regulated by estrogen, with testosterone playing a minor role compared to other hormones in females. Thus, the correct response highlights the specific functions of testosterone in male reproductive health and characteristics.