**Core Concept**
The onset of puberty in females is a complex process involving the hypothalamic-pituitary-gonadal axis, leading to the development of secondary sexual characteristics. The first sign of puberty in females is often a result of the increase in estrogen levels, which stimulate the growth and thickening of the vaginal and vulvar tissues.

**Why the Correct Answer is Right**
The first sign of puberty in females is typically the appearance of breast buds or thelarche, which is caused by the growth of breast tissue in response to estrogen. This is the earliest physical sign of puberty in females and is usually followed by the development of pubic hair (pubarche) and the growth of the uterus and vagina. The increase in estrogen levels also leads to the thickening of the vaginal and vulvar tissues, making them more sensitive to touch.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not a recognized first sign of puberty in females.
* **Option B:** Axillary hair is a secondary sign of puberty in females, appearing after thelarche.
* **Option C:** Menstruation (menarche) is a later sign of puberty in females, typically occurring after thelarche and pubarche.

**Clinical Pearl / High-Yield Fact**
The appearance of breast buds or thelarche is often the first sign of puberty in females, and it usually occurs around the age of 10-11 years.

**Correct Answer: C. Menstruation is not the first sign of puberty in females.