Not necessary, because “functioning well” is irrelevant to the reasoning. The author isn’t specific about the exact therapeutic value of cancer-support groups, so we have no basis to say that these groups can help people function well. In addition, nothing in the argument relies on the concept of “extreme stress.”

Not necessary, because even if disease IS a biochemical phenomenon, we still have evidence that participation in cancer-support groups reduces stress levels, and the author can still argue that this shows cancer-support groups might have some beneficial effects on health.

Necessary, becuase if it were not true — if stress CANNOT weaken the immune system — then we have no reason to think that cancer support groups have therapeutic value. The author brought up the reduction in stress level because she thought it had some relationship to the fact that a weakened immune system increases vulnerability to cancer. If stress doesn’t weaken an immune system, then there’s no relationship between those facts.

Too extreme to be necessary, because although we know that cancer support groups can reduce stress levels, that doesn’t commit the author to believing that these groups “eliminate” stress.

Not necessary, because even if stress is NOT a symptom of a weakened immune system, it can still cause a weakened immune system.