Hi j_shreyans,

Actually, the correct answer is D. This is a question that deals with cause and effect.

The immune system protects against MENTAL ILLNESS. The immune system protects against PHYSICAL ILLNESS.

But if a pre-existing MENTAL ILLNESS lowers immune system activity, then the conclusion (that the immune system protects against mental illness as well as against physical disease) fails. For the conclusion to hold, we must assume that low immune system activity causes low test scores. Answer D addresses this assumption because it eliminates the possibility that mental illness is causing low immune system activity.

This is an official GMAT question and it is a classic!!

There are dozens of imitation questions based on this one.

When dealing with cause and effect there are a couple of things to keep in mind.
1) When two items are correlated either of them can be the cause. For example, those persons who own laptop computers earn more money than those who do not. So it could be laptops = higher pay. Or those with higher pay can afford laptops.

In this case this is the same switch, right? It could be that mental illness weakens the immune system instead of a weak immune system leading to mental illness. So that is choice D.

2) And then you can have a third cause that is responsible for both effects. For example, the famous "daytime headlights" question. People who use headlights in the daytime average far fewer accidents than those who do not. Now it could be the headlights that cause the reduction in accidents, but it turns out to be a third factor - really careful drivers use the headlights and they also (of course) have fewer accidents.

That is a little about cause and effect. Remember, when they say that two things are correlated watch out for the argument concluded that one particular thing is the cause of the other.

Immune System Influences Psychiatric Disorders.People with higher rates of inflammation are more likely to show signs of depression than those with healthy immune function.
Is letter A safe to choose as an answer? High immune-system activity protects against mental illness better than normal immune-system activity