Final answer:
Interstitial cells, also known as Leydig cells, produce testosterone.
Step-by-step explanation:
Testosterone, an androgen, is a steroid hormone produced by Leydig cells, also known as interstitial cells. These cells are located between the seminiferous tubules in the testes. They are responsible for secreting testosterone, which plays a crucial role in the development of male sexual organs, physical changes during puberty, and the initiation of spermatogenesis. Therefore, the correct answer is D) testosterone.