Ans is D

Negation technique works here : Negation of assumption weakens conclusion.

(A' is negation of A )

A' : The longer people are married, the less happier and less healthier they become. : The duration of marriage is out of scope. This does not weaken the conclusion.

B' : Married couples who had a large, extravagant wedding are not happier than those who had a small,
simple ceremony. : different types of marriages do not weaken the conclusion.

C' : Married people can get depressed. : So what? This is the Most tempting answer. This extreme sentence does not weaken the conclusion that 'being married is good for one's health and attitude'. a depressed married person does not weaken the conclusion that, on average, married people are healthier and happier than unmarried people.

D' : Single people with depression or health problems are not as likely to get married as are other
single people. : This weakens the conclusion that marriage causes things. The negation of option D says that being happy and healthy makes a person more likely to get married.

E' : Some marriages are not more harmonious than others. : The conclusion is comparison between married and unmarried. So, irrelevant and does not weaken the conclusion.

Hope this helps.