The point at issue in this stimulus is whether this variation in moods exists, not why it exists.

The author doesn’t explain S.A.D. because they’re not even convinced it’s real.

As with (A), the author isn’t even convinced seasonal variation in moods is a real phenomenon.

A matching stimulus for (B) would say something like “perhaps people are more sad in the winter, but that’s just called being normal.”

What a well-crafted trap answer, eh? Very often when a study uses flawed methodology, the right answer is phrased “something something unrepresentative sample.”

Grammar is the key here – (C) says the population sample is unrepresentative. That is, the group of people chosen to take the survey doesn’t reflect the broader population (because, for instance, they only surveyed men or college students or whatever).

To describe the stimulus in unrepresentative sample terms (which the testwriters absolutely could have done), it’s the survey responses that are unrepresentative of people’s actual moods across time.

Here’s the psychologists’ assumption, which the author questions:

When asked to recall how they felt at various times in the past, people provide accurate answers.

(D) says the author attributes this assumption to the psychologists because all we’re given is the author’s summary of the psychologists’ position – according to the author’s telling, the psychologists make this assumption.

Lots of red flags in this answer. Demonstrating a fact like this would mean offering an alternative data set giving positive evidence of a different finding. The relative term fewer is also sus, because it requires two data points: psychologists think the number is [this], and the author thinks the number is [this lower number]. The temporal term previously is another one that needs specific support. It also demands two data points: in the past [blah], and now [blah].