My rephrasing of the argument:

Because low immune activity patients have lower mental health scores than normal/high activity patients, the immune system protects against mental illness as well as against physical disease.

A. This is wrong because the low is compared to normal/high in the argument. (High is not compared to normal.)

B. I see your point but this is out of scope (the body effects are irrelevant to the argument). A quick way to test is to negate. If the argument depends on an assumption, when you negate the assumption the argument should fall apart.

Even if "Mental illness is NOT similar to physical disease in its effects on body systems" the argument still holds up.

C. Out of scope.

D. If we negate, "Mental illness causes people’s immune-system activity to decrease" then the argument's conclusion is flawed because it illness causes low immune activity, rather than low immune activity causing illness. So this assumption is necessary for the argument to hold up.

E. Irrelevant

Does this help?

My intuition whispers that it is A.