**Core Concept**
The initial sign of sexual maturity in boys is a significant milestone in puberty, marking the beginning of physical and hormonal changes. This process is primarily driven by the **hypothalamic-pituitary-gonadal axis**, where the hypothalamus stimulates the pituitary gland to release hormones that trigger testosterone production in the testes.

**Why the Correct Answer is Right**
The correct answer is related to the first noticeable change in boys during puberty. Testosterone surge leads to various physical changes, but the initial sign is often the **enlargement of the testicles**, followed by changes in the texture and color of the scrotum. However, among the given options, the closest related to an early sign of puberty in boys, considering common manifestations, would be related to physical changes that are easily noticeable, such as voice changes or other secondary sexual characteristics.

**Why Each Wrong Option is Incorrect**
**Option A:** "Fat spurt" is not typically the initial sign of sexual maturity in boys; it's more related to the growth spurt seen in both genders during puberty.
**Option B:** Development of Adam's apple, or **laryngeal prominence**, is indeed a sign of puberty but not usually the first.
**Option D:** Appearance of facial hair is also a sign of puberty but tends to occur after other changes have begun.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the sequence of pubertal events can vary among individuals, but generally, testicular enlargement is the first sign, followed by other changes.

**Correct Answer:** B. Development of Adam's apple