(A) Older adults generally have slower metabolisms and hence find it more difficult to lose weight regardless of diet plan. - this tells that the older adults have difficulty in losing the weight but not helping to understand why the success rates are identical in young individuals and older adults, hence, ELIMINATE.
(B) Younger adults tend to be more physically active, which can enhance the effectiveness of any diet plan. - this answer choice is just concerned with the young individuals and not giving any information about the older adults, hence, ELIMINATE
(D) The new high-protein diet plan was primarily marketed towards individuals who have not previously tried structured dieting. - we are not concerned with "to whom the diet plan was marketed", we are concerned to find out the reason behind the identical success rates which this answer choice is not helping to understand. ELIMINATE
(E) Most diet plans do not consider age-related metabolic changes when calculating recommended food intakes. - Reason to eliminate this option is "most", not because it is a superlative but it is saying about the diets in general that "most" diets do not consider age-related metabolic changes...This is not helping us answer the question about the reason behind the identical success rate and also the diet plan that is discussed in the passage, it might or might not be in the "most" that this option is talking about and still it will not help in answering the question where we are concerned about finding the reason behind the identical success rate. ELIMINATE
(C) Individuals who feel they have struggled significantly with weight for many years are more likely to adhere strictly to new diet recommendations. - Given in the passage: "older adults who generally struggle with weight loss". Based on this we can infer that since the older adults struggled with the weight loss for many years, they are more likely to adhere strictly to the new diet recommendations and this is the reason behind the identical success rates. This is the best answer choice among all the given options.
Hence, Option (C) is the right answer.