Final answer:
Having a sibling with an eating disorder increases one's risk for developing an eating disorder due to genetic and environmental factors, including a family history of such conditions.
Step-by-step explanation:
The correct answer to the question of which factor increases one's risk of developing an eating disorder is C) Having a sibling with an eating disorder. Eating disorders often have a genetic component, where a close biological relative with an eating disorder can significantly increase the likelihood of an individual developing a similar condition. In addition to genetic factors, various environmental factors can contribute to the increased risk, including child abuse, parental control over eating habits, a fragile sense of self-identity, and social isolation. Cultural pressures, particularly the idealization of thinness, also play a role in the development of eating disorders, most notably anorexia nervosa and bulimia nervosa.