Female athletes who previously had regular menstrual periods but have stopped menstruating are experiencing

Amenorrhea

Explanation:

Amenorrhea is a condition in which menstruation periods (in females) become irregular. It is a direct result of overtraining and lack of rest. It is common in many athletes and is caused by excessive or lack of hormones that are also depleted during strenuous activities. Women are encouraged to take a rest once the onset of amenorrhea occurs as this may lead to complications in conceiving and pregnancy. Amenorrhea is also caused by the lack of necessary nutrients in the body due to poor nutrition.

c would be the answer