This is a necessary assumption question, so that tells me we'll have to really analyze the conclusion being made, and the evidence provided to support that conclusion and consider the gap, or "concept shift."

-Married ppl HAPPY & HEALTHY, according to research
-Conclusion: Marriage (+) health/attitude

The concept shift here is that we're introducing a general conclusion ("being married is good") based on specific information ("recent research").

It is assumed that this research is valid and can be used as a basis for such a far-reaching, widespread conclusion. I'd expect the correct answer to back up the idea that single ---> married = happiness.

The reason (D) is correct is because it REMOVES a possible weakening idea: married people were ALREADY happy/healthy when they were single. If ONLY happy/healthy singles get married, then OF COURSE the research would show married people are happier/healthier, but NOT NECESSARILY due to the Conclusion ("marriage is good"). (D) removes another explanation for the research, making it more likely that the Conclusion is true.

Hello!

This problem seems to be properly explained in posts above, but I want to have a go at it too)

First of all, what are facts (things that are true) and conclusion (just claims that can be questioned) of this argument? The sentences about research and media reports may seem to convey the same idea, but they are not. All we have to do is to look closely at them.

"Recent research has indicated that..." --> more or less we can assume that we're dealing with facts here. What are these facts? That married people are happier and healthier than unmarried people. Note: married people can be happier and healthier because of a lot of reasons. Maybe people who get married inclined to singing and telling jokes about marriage and that makes them healthy and happy. We don't know the reason. Of course, it's easy to assume (and don't even notice that we are only assuming and don't know for sure) that the marriage itself is what makes them happy and healthy.

The conclusion is presented here through reports of commentators that "being married is good for one's heath and attitude". As Rich pointed out, it means that marriage actually makes people healthier and happier.

When dealing with assumption questions the easiest way to me is to go through all the choices and try "assumption properties" on all of them. "Assumption properties" are:
1) When the assumption is true, it doesn't refute the conclusion of the argument. Usually (and here it is the case too) all the answer choices seem to satisfy this condition.
2) When assumption is false, the conclusion is crushed down. Only D fits here. If unhappy and unhealthy single people are less* likely to get married, then more people who are originally inclined to be healthy and happy are getting married, and as a result we have more married people who are healthy and happy.
*"Just as likely" in fact has two opposites: more likely or less likely. If we use "more" instead of "less" in the paragraph above, the method won't work. But it's enough if at least one of two possible options works. [/i]

If marriage makes people healthier, then people who marry should report better overall health and less illness than when they were single.People who married reported slightly worse health than they had when they were single. Get Married, Get Healthy? i don't think so..