**Core Concept**
The onset of puberty in boys is marked by a series of physiological changes, including the development of secondary sex characteristics. These changes are triggered by the surge in production of sex hormones, primarily testosterone. The enlargement of testes is one of the earliest and most reliable signs of puberty in boys.

**Why the Correct Answer is Right**
The enlargement of testes, also known as testicular enlargement, is a critical indicator of the initiation of puberty in boys. This occurs due to the increased production of testosterone by the Leydig cells in the testes. Testosterone stimulates the growth of the testicles, leading to an increase in their size and weight. The enlargement of testes is usually the first sign of puberty, often occurring between the ages of 9 and 14. This change is a result of the activation of the hypothalamic-pituitary-gonadal axis, which regulates the production of sex hormones.

**Why Each Wrong Option is Incorrect**
**Option A:** Enlargement of the penis is a sign of puberty, but it typically occurs after the enlargement of testes. The penis begins to grow in length and circumference during puberty, but this change usually occurs after the testicular enlargement.
**Option C:** The appearance of pubic hair is another sign of puberty, but it is not the earliest sign. Pubic hair growth usually occurs after the enlargement of testes and the growth of the penis.
**Option D:** The appearance of axillary hair is a sign of puberty, but it is not as early a sign as testicular enlargement. Axillary hair growth typically occurs after the onset of puberty, but it is not as reliable an indicator as testicular enlargement.

**Clinical Pearl / High-Yield Fact**
It is essential to note that testicular enlargement is a sensitive indicator of puberty, and any delay in this change may indicate an underlying hormonal or genetic issue. A thorough evaluation by a pediatrician or endocrinologist is necessary to rule out any underlying conditions.

✓ Correct Answer: B. Enlargement of testes