Answer choice C suggests that the psychologist's survey use an unrepresentative sample. However, the author makes no such claim.
Rather, the author disagrees with the psychologists on the grounds that people in the survey may not be able to accurately recall their own past mental states. This is not saying that the people are somehow different from the general population, but rather it is to say that people in general are not fully able to remember how they felt.
The assumption that D refers to is assuming that people ARE able to remember their past mental states, which the author disagrees with.