**Core Concept**
Spermatogenesis is a complex, highly regulated process of male germ cell development, resulting in the production of mature spermatozoa. It occurs in the seminiferous tubules of the testes and is influenced by a delicate balance of hormones, including testosterone and follicle-stimulating hormone (FSH).

**Why the Correct Answer is Right**
The correct answer involves the understanding of the role of FSH in spermatogenesis. FSH is secreted by the anterior pituitary gland and stimulates the Sertoli cells in the seminiferous tubules to support the development of sperm cells. FSH binds to its receptor on the Sertoli cells, triggering a cascade of intracellular signaling events that promote spermatogenesis. This process is crucial for the normal development of sperm cells, and any disruption in FSH signaling can lead to impaired spermatogenesis.

**Why Each Wrong Option is Incorrect**
**Option A:** is incorrect because it does not accurately describe the role of FSH in spermatogenesis. While FSH is essential for spermatogenesis, it is not the primary hormone responsible for the initiation of spermatogenesis.

**Option B:** is incorrect because it incorrectly identifies the primary hormone responsible for spermatogenesis. Testosterone is indeed essential for spermatogenesis, but it is not the primary hormone responsible for initiating the process.

**Option C:** is incorrect because it does not accurately describe the role of FSH in spermatogenesis. FSH does not directly stimulate the Leydig cells to produce testosterone.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that FSH is a key regulator of spermatogenesis, and any disruption in FSH signaling can lead to impaired spermatogenesis. This is why FSH levels are often used as a diagnostic tool to assess male fertility.

**Correct Answer:** D.