Final answer:
The most distinguishing symptom of rumination disorder is the regurgitation and rechewing of food, differentiating it from other disorders like binge eating disorder and bulimia nervosa. The correct answer is option d).
Step-by-step explanation:
The most distinguishing symptom of rumination disorder is d) Regurgitation and rechewing of food. Rumination disorder is a feeding and eating disorder where an individual regurgitates food they have previously chewed and swallowed, rechews it, and then either reswallows or spits it out. This condition is distinct from other eating disorders such as binge eating disorder and bulimia nervosa, which involve different patterns of eating behavior and inappropriate compensatory behaviors like purging.