Which of the following is a risk factor for breast cancer?

Early onset of menstruation, or menarche before age 12, is a recognized risk factor for breast cancer due to its association with prolonged exposure to estrogen. Estrogen plays a significant role in the development of breast tissue and can increase the risk of cellular changes that may lead to breast cancer over time. The earlier a woman begins menstruating, the more years she is exposed to estrogen, which can contribute to an increased lifetime risk of developing breast cancer.

In contrast, regular exercise is generally associated with a lower risk of breast cancer as it can help maintain a healthy weight and reduce estrogen levels. A high-fat diet has an ambiguous relationship with breast cancer risk, and recent studies indicate that while diet can influence overall health, it is not a definitive risk factor for breast cancer. Similarly, a low body mass index (BMI) is often associated with lower levels of estrogen production, particularly after menopause, and is typically not considered a risk factor for breast cancer.