Final answer:
Estrogen and testosterone levels contribute significantly to the difference in body mass between men and women.
Step-by-step explanation:
The factor that contributes significantly to the difference in body mass between men and women is a) Estrogen and testosterone levels. Men have higher levels of testosterone, which promotes muscle growth and higher basal metabolic rate. Women have higher levels of estrogen, which promotes the deposition of fat in areas such as the hips, thighs, and breasts.