the argument is as follows.
Evidence: Married people are happier and healthier
Conclusion: So marriage is good for health and attitude.

One way to identify the answer is to negate the options and see if the conclusion still holds true.

A. The longer people are married, the happier and healthier they become.
Suppose people don't become healthier and happier, the longer they are married. Suppose they stay just happy and not happier with time. Still the conclusion that marriage may lead to people being happy, holds true.

(B) Married couples who had a large, extravagant wedding are happier than those who had a small, simple ceremony.
Suppose even people who got married in a simple ceremony is as happier as people who got married extravagantly. Still marriage leads to happiness and conclusion holds.

(C) Married people cannot get depressed.
Suppose married people get depressed sometimes. But still they may be happier at other times as compared to singles and so conclusion is unchanged.

(D) Single people with depression or health problems are just as likely to get married as are other single people.
Suppose single people with depression or health problems don't get married usually. Then it means mostly, the lively and healthiest of people get married and therefore it may result in more married people being healthy and happy. This disproves conclusion by contradicting the basic assumption and instead indicating that the marriage as an institution may not be entirely responsible for a happier and healthy life.

(E) Some marriages are more harmonious than others.
Suppose all marriages are equally harmonious. So what? It talks only about the relative level of harmonious relations in marriages. IT does not talk about differences between married and unmarried people. It is out of scope and does not change the conclusion.

So the BEST FIT is D