**Core Concept**
The onset of puberty in males is marked by the activation of the hypothalamic-pituitary-gonadal axis, leading to the development of secondary sexual characteristics. Testicular growth is a key indicator of pubertal development, and its size serves as a reliable marker of testicular function.

**Why the Correct Answer is Right**
During puberty, the testes undergo significant growth, increasing in size and weight. This growth is primarily driven by the production of gonadotropin-releasing hormone (GnRH) from the hypothalamus, which stimulates the release of luteinizing hormone (LH) from the anterior pituitary gland. LH, in turn, binds to its receptor on Leydig cells, stimulating the production of testosterone, the primary male sex hormone responsible for testicular growth and development. By the onset of puberty, testicular size typically reaches 4 mL, indicating the completion of testicular development.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because testicular size at birth is approximately 1-2 mL, and it increases significantly during puberty.
**Option B:** This option is incorrect because the average testicular size in an adult male is around 15-20 mL, much larger than the size at the onset of puberty.
**Option C:** This option is incorrect because it does not provide a specific size, making it an ambiguous answer.

**Clinical Pearl / High-Yield Fact**
A useful clinical correlation is that the testicular size can be used as an indicator of pubertal development. A testicular size of 4 mL or more is generally considered a reliable marker of testicular function and pubertal completion.

**Correct Answer: C. 4 mL**