Selection (C) is correct. The assertion (a) is true; however, the reason (r) is false.
Individuals with eating disorders such as anorexia nervosa and bulimia nervosa are indeed preoccupied with food, dieting, and body weight, often displaying a profound fear of weight gain and a distorted body image.
Nevertheless, these disorders are more frequently identified in adolescents and young adults, not predominantly in older adults. Although adults can experience these conditions, their incidence is higher among teenagers and young adults, rendering the reason statement inaccurate.