**Core Concept**
Spermatogenesis is the process by which immature cells in the testes develop into mature sperm cells. This complex process is regulated by a balance of various **hormones**, including **gonadotropins** and **sex steroids**. The primary hormones involved in maintaining spermatogenesis are **follicle-stimulating hormone (FSH)** and **testosterone**.

**Why the Correct Answer is Right**
The correct answer involves **FSH** and **testosterone**, as these hormones play crucial roles in the regulation of spermatogenesis. **FSH** stimulates **Sertoli cells** in the testes to support the development of sperm cells, while **testosterone**, produced by **Leydig cells**, is essential for the initiation and maintenance of spermatogenesis. The presence of both **FSH** and **testosterone** is necessary for the completion of spermatogenesis.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it lacks the essential hormone **testosterone**.
**Option B:** Incorrect as it only mentions one aspect of the hormonal regulation.
**Option C:** Incorrect because it does not include **FSH**, which is vital for Sertoli cell function.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **hypogonadotropic hypogonadism** (lack of **gonadotropin-releasing hormone (GnRH)**, **FSH**, and **luteinizing hormone (LH)**) can lead to impaired spermatogenesis due to the decreased production of **testosterone**. Understanding the interplay between these hormones is crucial for diagnosing and treating male infertility.

**Correct Answer:** D. FSH and Testosterone.