A - lean young adults. B - people who live long. C - adults who gained one pound every year.
Premise: B tend to have been (usually are) A who became C. B=A+C
Conclusion: A can become B if they become C too. A-> B, if A+C

A. reasons for the truth of conclusion (premise) presupposes (requires) the truth of conclusion. No, premise does not require the conclusion to be true. Rather, the conclusion is clearly derived from the premise.
B. there is no guarantee in the argument anywhere. 'Tends to' does not indicate a guarantee.
C. Proof is provided. Also, it is unclear which two phenomena occur together and which one is the cause? A and B do not occur together. There is a 'if' relationship between A and B which makes them sequential.
D. Correct. Concludes that combination of A and C causes B, whereas 'tends to' in the premise indicates only a weak association. So does 'can improve' in conclusion.
E. The conclusion restricts itself to lean people. Population as a whole is not referred to anywhere in the argument.