Amenorrhea may occur among women at fat levels as high as:

The correct answer is "16%"

16% of body fat in women is considered as low as normal body fat levels in women normally ranges from 22% to 25%. In cases of low body fat levels (i.e. in athletic women), there will be relative amenorrhea because of the suppression of the follicle-stimulating hormone and the luteinizing hormone leading to a type of amenorrhea called athletic amenorrhea.

~potters6/te801/biology/biounits/respiration.htm