Final answer:
The average length of the menstrual cycle is D) 28 days, but it can normally vary from 21 to 45 days. Menstruation typically lasts about five days, but can also vary in duration.
Step-by-step explanation:
The average length of the menstrual cycle is D) 28 days. However, it's important to understand that this average can vary among different individuals and also from one cycle to the next within the same individual. The menstrual cycle's length may range typically from 21 to 45 days, and it is considered regular if the longest and shortest cycles differ by less than eight days.
The menstrual cycle begins on the first day of menstruation and ends when the next period begins. The ovarian cycle and the menstrual cycle work concurrently, with the ovarian cycle managing the release of eggs and the menstrual cycle preparing the uterine lining. Both are coordinated over a typical range of 22-32 days, again with an average of 28 days.