Psychologist: Some psychologists mistakenly argue that because dreams result from electrical discharges in the brain, they must be understood purely in terms of their physiological function.
Main Conclusion (mistakenly - opinion)
Psychologists mistakenly argue that dreams must be understood purely in terms of their physiological content (because they result from electrical discharges).
i.e., dreams can be understood in non-physiological terms as well
Intermediate Conclusion
Electrical discharges cannot completely explain the phenomenon of dreaming.
Evidence (since)
Dream content varies enormously.
Evaluate
We're being asked about a claim that is prefaced by since, so it's a guarantee that it's performing the role of supporting the other half of its sentence. (FABS = for, after all, because, since = Support Indicators)
Goal
Let's look for something like Premise / Support / Evidence for the claim that .
If they're getting mean, we may need to worry about whether that final claim is the main conclusion or just an intermediate one, but we won't worry about that until we see it.