While it was once believed that the sort of psychotherapy appropriate for the treatment of neuroses caused by environmental factors is also appropriate for schizophrenia and other psychoses, it is now known that these latter, more serious forms of mental disturbance are best treated by biochemical—that is, medicinal—means.
Conclusion (this is conclusive evidence)
Psychoses, unlike neuroses, have nothing to do with environmental factors but rather are caused by some purely organic condition (internal stuff).
Evidence
It was once believed that the appropriate psychotherapy for neuroses caused by environmental factors would also work for psychoses.
But now we know that these psychoses are best treated by biochemical (medicinal) means.
Evaluate
This is a pretty dense stimulus to parse, but ultimately this argument is going to come down to an assumption that .
The evidence and the conclusion are both doing a comparison/contrast between neuroses and psychoses.
We had neuroses that were caused by environmental factors (i.e. your mom used to always lick her finger and fix your bangs, and now you're super neurotic about fixing your hair). The appropriate treatment was psychotherapy (i.e. sitting with a therapist and talking through your problems, trying to get your mind / psyche into better shape).
We used to believe that this psychotherapy was also the right treatment for psychoses (such as multiple personality disorder). But now we know that a better treatment is pharmaceutical drugs (i.e. stuff you ingest that changes your body chemistry ... and maybe gets your brain into better shape).
Because we can better treat psychoses with something (drugs) that changes your biochemistry than with something (psychotherapy) that changes your mind / thoughts, the author is concluding that psychoses must be more about the body and less about the mind. Thus, they are more caused by body stuff than by environmental factors.
Goal
Let's look for an answer that speaks to this weird move from "better treatment" to "the root cause".
The author thinks that a condition that is better treated by changing brain chemistry than by changing mental activity must therefore be caused by something wrong with brain chemistry, rather than something wrong with environmental factors that messed up someone's thoughts.
Since this question stem is putting "ignores the possibility" in front of every answer choice, we're really just going to ask ourselves whether each answer choice Weakens.
Thus, we could call this a Flaw question, or we could call it a Weaken (that's disguised as Flaw).