+1 D

In the first sentence of the argument, the author shows a correlation between inmune-system activity and mental health. But then, in the conclusion indicates that there is a CAUSAL relation between those elements. (Notice the word "protects"). In other words, inmune-system activity is the cause, and mental health is the effect. It's clear that the author is assuming that there is a causal relationship in the described order.

It seems that you don't have clear some concepts of CR. Have you tried this book: CR Powerscore Bible?
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can any expert post a comprehensive soln of this cr. thanx in advance.

Argument:
Researcher discovery from experiment: people with low levels of immune-system activity tend to score much lower on tests of mental health than do people with normal or high immune-system activity.
Researcher conclusion: the immune system protects against mental illness as well as against physical disease.

Pre thinking:
Researcher observation is that low levels of immune-system activity tend to have lower score on mental health.
He made the conclusion that immune system protect against mental illness.
We can make another conclusion that lower score on mental health (i.e. mental) illness causes lower immune system activity. So an underlying assumption here is that mental illness does not cause low immune system activity.

Analysis of answer Choices:
(A) High immune-system activity protects against mental illness better than normal immune-system activity does.
INCORRECT: the argument is not about high and normal immune system. Nothing about normal immune system is provided.

(B) Mental illness is similar to physical disease in its effects on body systems.
INCORRECT: The argument is focussed on mental illness and immune system and conclusion has nothing to do with body systems in general.

(C) People with high immune-system activity cannot develop mental illness.
INCORRECT: Conclusion says that immune system protects against mental illness like it does again physical illness. Doesn’t mean that people with high immune system can’t develop mental or physical illness.

(D) Mental illness does not cause people's immune-system activity to decrease.
CORRECT: Look at the pre thinking. If this is false then we will get the conclusion that lower score on mental health (i.e. mental illness) causes lower immune system activity. So for researchers conclusion to hold this must be true and be the assumption.

(E)Psychological treatment of mental illness is not as effective as is medical treatment.
INCORRECT: This doesn’t tell if immune system is effective for preventing mental illness or not.

The key here was that researcher could have made multiple conclusion from his experiment.Why he reached at one conclusion helped me in determining the underlying assumption.
Hope this helps.
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The Kudo please

Mental health and mental illness can be taken as same thing. I mean a lower mental health will mean some mental illness.
So we can consider lower score on mental health = mental illness.
However this is not the main point of the argument.

Just think only about the following statement:
Researcher observation is that low levels of immune-system activity tend to have lower score on mental health.

What conclusion will you make out of it?

I will make one of the following two conclusions:
1. EITHER – Lower immune system causes mental illness which means that immune system protect against mental illness.
2. OR - Mental illness causes lower immune system.

Now author chooses 1st as his conclusion and ignores the second .So we will have to find a way to show that second conclusion will not hold and that will be our assumption.
This is what choice D do and hence the correct answer.

Hope it is clear now.
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The Kudo please