IMO D.

I was caught in two choices, B and D.
Here's how I proceeded.
The argumet begins by mentioning that 1pound/year weight gain results in long life. This sets up a causal premise. Based on this, the author concludes that the reverse is true, which is a common flaw.
The author has only established a relation between the two. the cause->effect relationship has not been established properly.

IMO D

Premises:
People who live unusually long tend to have been lean young adults who went on to gain approximately one pound every year
Conclusion:
lean young adults can improve their chances of living a long life by gaining about a pound every year.

(A) gives reasons for the truth of its conclusion that presuppose the truth of that conclusion
This option says that premise supports conclusion and viceversa. But, as per passage conclusion is not supporting the premise. Both are different.
(B) proceeds as though a condition that by itself is enough to guarantee a certain result must always be present for that result to be achieved
There is no condition specified here.
(C) assumes without proof that two phenomena that occur together share an underlying cause
Argument doesnt say that both weight gain and long life are occuring together. Moreover, the causes for both may be different.
(D) concludes that one phenomenon is the cause of another when at most what has been established is an association between them
Looks perfect. Here cause is Gaining a pound a year. Effect is chances of living longer.
There is just association between them.
(E) fails to recognize that a tendency widely shared by a subgroup within a given population will not necessarily be widely shared by that population as a whole
"population as a whole" - this makes this answer wrong. Argument discusses only about "lean young people" both in premise and conclusion.

Hope this helps!!

kvcpk wrote:IMO D

Premises:
People who live unusually long tend to have been lean young adults who went on to gain approximately one pound every year
Conclusion:
lean young adults can improve their chances of living a long life by gaining about a pound every year.

(A) gives reasons for the truth of its conclusion that presuppose the truth of that conclusion
This option says that premise supports conclusion and viceversa. But, as per passage conclusion is not supporting the premise. Both are different.
(B) proceeds as though a condition that by itself is enough to guarantee a certain result must always be present for that result to be achieved
There is no condition specified here.
(C) assumes without proof that two phenomena that occur together share an underlying cause
Argument doesnt say that both weight gain and long life are occuring together. Moreover, the causes for both may be different.
(D) concludes that one phenomenon is the cause of another when at most what has been established is an association between them
Looks perfect. Here cause is Gaining a pound a year. Effect is chances of living longer.
There is just association between them.
(E) fails to recognize that a tendency widely shared by a subgroup within a given population will not necessarily be widely shared by that population as a whole
"population as a whole" - this makes this answer wrong. Argument discusses only about "lean young people" both in premise and conclusion.

Hope this helps!!

what level would you rate this question ?

nikhilkatira wrote: what level would you rate this question ?

Well.. I am not an expert.. But, I heard that flaw in the reasoning and Method of reasoning questions appear only when we are doing quite good in the exam. So should be a 700+ question.