A researcher discovered that people who have low levels of immune-syst

Understanding the argument -
A researcher discovered that people who have 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. - Fact. Presents a correlation between people with a "low" level of the immune system and a "normal or high" level of the immune system. People with "low" levels of immune system score lower on tests (of mental health) than the people with "normal or high" levels of immune system.

The researcher concluded from this experiment that the immune system protects against mental illness as well as against physical disease. - Conclusion. So, the author converts the correlation to causation in this sentence by saying that the "immune system" protects against "mental illness," and he/she adds "physical disease as well."

So, if we say A and B are correlated in the statement, and the conclusion tries to establish one relationship, say A causes B, then we need to assume that "Z did not cause B or Z did not cause A and B, and B did not cause A. Because three relationships are possible - A causes B, B causes A, and Z causes B.

Option Elimination

(A) High immune-system activity protects against mental illness better than normal immune-system activity does. - In the argument, "high or normal" is one category compared with "low." The argument does not compare "normal" and "high". Moreover, how is it even a minimum condition or missing premise for the conclusion, "the immune system protects against mental illness as well as against physical disease." Out of scope? Yes.

(B) Mental illness is similar to physical disease in its effects on body systems. - These two words appear in the conclusion, but the similarity between the two is not the minimum condition for the conclusion to hold. Distortion.

(C) People with high immune-system activity cannot develop mental illness. - The conclusion says the "immune system" protects against "mental illness." It can be "normal or the high" immune system. See, the conclusion is trying to establish "Causality" between the "immune system," which can be "normal or high," and "mental illness." The conclusion doesn't say that having an "immune system" is a sufficient condition for protection against "mental illness" or that having an "immune system" guarantees protection against "mental illness." Right? Yes. Mental illness is complex and multifaceted, influenced by various factors beyond just the immune system, such as genetics, environment, lifestyle, and more.

But this option is trying to establish a sufficient conditional relationship to guarantee that one cannot develop mental illness if one has a high immune system. This is not the scope of the argument. The scope of the argument is to establish causality, for which we need to assume that B did not cause A or Z did not cause B. Establishing sufficiency is not even the scope of the argument.

There is an easier way to eliminate this (though not foolproof, so use it cautiously). What is an assumption? Is it a missing premise? What is the premise? It's a Fact. What is this statement? It's an opinion. Wrong.

(D) Mental illness does not cause people’s immune-system activity to decrease. - good. It says B did not cause A. ok.

(E) Psychological treatment of mental illness is not as effective as is medical treatment. - Out of scope.