**Core Concept**
Precocious puberty in girls refers to the early onset of pubertal changes, typically before the age of 8 years. This condition is characterized by the premature activation of the hypothalamic-pituitary-gonadal axis, leading to the development of secondary sexual characteristics.

**Why the Correct Answer is Right**
The correct answer is 8 years because the Tanner scale defines the onset of breast development, a hallmark of puberty, as occurring at approximately 8 years of age in girls. According to the American Academy of Pediatrics, breast development before the age of 8 years is considered precocious puberty. This early onset is due to the premature activation of the hypothalamic-pituitary-gonadal axis, which stimulates the growth of ovarian follicles and the production of estrogen. Estrogen, in turn, promotes the development of secondary sexual characteristics, such as breast growth, pubic hair, and vaginal bleeding.

**Why Each Wrong Option is Incorrect**
**Option B:** 10 years is considered a normal age for breast development in girls, according to the Tanner scale. Therefore, breast development at this age is not considered precocious puberty.
**Option C:** 9 years is close to the normal age for breast development, and some girls may develop breast tissue at this age without being considered precocious.
**Option D:** 11 years is also a normal age for breast development in girls, and development at this age is not indicative of precocious puberty.

**Clinical Pearl / High-Yield Fact**
It's essential to note that breast development in girls can be influenced by various factors, including genetics, nutrition, and environmental exposures. A thorough evaluation by a pediatric endocrinologist is necessary to determine the underlying cause of precocious puberty and to develop an appropriate treatment plan.

**✓ Correct Answer: A. 8 year**