This is a STRENGTHEN the argument question so let's start by analyzing the core.
Therapeutic intervention before onset of mental disorders can mitigate contributors to them
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Research is needed to both (1) verify these results and (2) design health care measures
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We should increase funding for research
This is a very subjective conclusion, i.e. that we "should" do something. How do we strengthen a claim like this? Typically we do this by showing that what the author claims we "should" do - in this case, research - is actually worth doing. The correct answer will typically show some kind of benefit to that thing that we "should do."
(A) This is comparing minor mental disorders with minor health problems. I just don't know how this is irrelevant and it certainly doesn't help the conclusion.
(B) This is just giving more detail about how research will be conducted. We don't need to know anything about that. We just need to know if this research is even worth doing.
(C) This one looks pretty good! It is saying that reducing the risk factors, i.e. those things that would be reduced if we could do more therapeutic intervention, is pretty cheap in comparison to actually treating the mental disorders long-term. This provides a reason why the research is worth doing!
(D) That's great but it doesn't help answer why we should increase funding! In fact, it perhaps provides a reason for not increasing the funding - after all, it is already really high!
(E) This is kind of irrelevant. This is talking about what happens after someone gets a mental disorder - we want to focus on what happens before.
(C) is obviously the best answer here and the only one that strengthens. If we accept (C), we have a good reason to increase funding for research because, if we do so, it will allow us to help mitigate mental disorders before they happen - thus saving money in the long run.