Final answer:
Menarche signals a girl's first menstrual period, marking the onset of puberty and the beginning of reproductive capability. It typically occurs around 12 years of age but can range from 8 to 16 years. In Amelia's case, at 12 years old, she has begun experiencing menarche, indicating her entry into her reproductive years.
Step-by-step explanation:
Menarche refers to a girl's first menstrual period, which is a major event during puberty. It signifies the time when her reproductive system has matured enough to begin the menstrual cycle. In the United States, the average age at which girls experience menarche is approximately 12.4 to 12.5 years old, but it can occur anytime between the ages of 8 and 16 and still be normal. Menarche indicates the start of fertility periods but not fertility at its peak, which generally occurs in the twenties, and is different from menopause, which is the cessation of the menstrual cycles.
So, when Amelia, who is 12 years old, has recently experienced menarche, it means she has gone through option B. the first menstrual period. This event is a natural part of development for females and is influenced by a combination of genetic and environmental factors.