find the assumption

What we need to look at when dealing with this question is the connection between mental illness and the immune system. The author argues that the low level of immune-system activity might be the REASON for mental illness. Now we need to find an assumption that will support this claim.
A - this is not an assumption because this answer basically repeats the claim stated in the conclusion - if you reverse this sentence, you will have: Low immune-system activity provides worse protection against mental illness than normal immune-system activity does
B - out of scope, not mentioned
C - again, this is the repetition of the paraphrased information from the conclusion
E - out of scope
D - correct, since this answer implies that the level of immune-system activity is the same before and after somebody starts to suffer from a mental disorder, meaning that the low immunity could have been the CAUSE and not the EFFECT

For cause and effect type of reasoning there are some well defined rules.

(1) if Cause ------> effect then effect ----X---> cause

(2) if Cause ------> effect then NOT cause ------> NOT effect

(3) if Cause 1 -----> effect then any other Cause CAN NOT HAVE effect

The choice D is the example of (1)