**Core Concept**
The onset of puberty in females is triggered by a surge in estrogen levels, which is mediated by the hypothalamic-pituitary-gonadal (HPG) axis. The growth of pubic hair is a key indicator of pubertal development, and it is influenced by the interaction between hormones and the skin.

**Why the Correct Answer is Right**
The correct answer is **Estrogen**. Estrogen stimulates the growth of pubic hair by binding to estrogen receptors in the hair follicles, leading to an increase in the size and pigmentation of the hair. This process is mediated by the enzyme aromatase, which converts androgens to estrogens in the skin. As estrogen levels rise during puberty, the growth of pubic hair becomes more pronounced.

**Why Each Wrong Option is Incorrect**
* **Option A:** **Insulin-like Growth Factor 1 (IGF-1)** is involved in growth and development, but it is not directly responsible for pubertal growth in females.
* **Option B:** **Growth Hormone (GH)** is important for overall growth and development, but it does not specifically stimulate the growth of pubic hair.
* **Option D:** **Progesterone** is involved in the menstrual cycle and pregnancy, but it is not the primary hormone responsible for pubertal growth in females.

**Clinical Pearl / High-Yield Fact**
The growth of pubic hair is a key indicator of pubertal development, and it typically begins around 12-13 years of age in females. The timing and progression of pubic hair growth can provide important clues about the underlying hormonal status and the presence of any underlying medical conditions.

**Correct Answer: C. Estrogen**