Final answer:
Menarche marks the onset of the menstrual cycle, serving as the first menstrual period and indicating the start of a female's reproductive years, usually beginning during puberty.
Step-by-step explanation:
The onset of the menstrual cycle is termed menarche, which is option 2 in the list provided. Menarche represents a significant milestone in a female's reproductive development, marking the beginning of menstruation.
Menarche generally occurs during puberty and is characterized by the first menstrual period. In the United States, the average age of menarche is approximately 12.5 years, although it can normally occur anywhere from the ages of eight to 16. The menstrual cycle includes several phases, but menarche specifically refers to the commencement of the monthly cycle that will continue until menopause, typically in middle adulthood.
Understanding menarche is important in the broader context of female reproductive health, as it signals the start of fertility and the ability to conceive. The menstrual cycle is marked by the cyclic release of an egg from the ovaries and the preparation of the uterus to potentially sustain a pregnancy.