The psychologists assume that the sample surveyed can accurately report their past psychological states during seasons. Negate and argument falls apart, so, IMO, (D).

I believe it is (D).

The psychologists assume that the data(opinion of the people) is correct to obtain a correlation. The authur questions the validity of the data hence questions the validity of the assumption.

1) Psychologists have claimed that many people are more susceptible to psychological problems in the winter than in the summer
2) Psychologists call this condition seasonal affective disorder.
3) Their claim is based on the results of surveys in which people were asked to recall how they felt at various times in the past.
4) However, it is not clear that people are able to report accurately on their past psychological states.
5) Therefore, these survey results do not justify the psychologists’ claim that there is any such condition as seasonal affective disorder. <-- conclusion

The author criticizes the psychologists claim by

(A) offering an alternative explanation of the variation in the occurrence of psychological problems across seasons
- no. It criticizes the claim by debunking the accuracy of the accounts of the people surveyed

(B) questioning whether any seasonal variation in the occurrence of psychological problems could properly be labeled a disorder
- No.

(C) questioning the representativeness of the population sample surveyed by the psychologists
- The author isn't questioning if the sample can be representative.

(D) questioning an assumption that the author attributes to the psychologists
- This is the answer choice. The author challenges the assumption that the psychologists made (assumed that the people can accurately recall their psychological state)

(E) demonstrating that fewer people actually suffer from seasonal affective disorder than psychologists had previously thought
- No such demonstration stated in the passage

D it is.

Nice post,This is something which i really didn't knew before.May be they are right i mean what they have claimed that can be right.
May be it effects badly in winters or get worse.Keep sharing your opinions people.