Compared with their less happy peers, the happiest 20-year-olds are subsequently likely to marry and likely to divorce.

D). More; less

Explanation:

As per the question, 'in comparison to the lesser happy companions, the happier ones have 'more' probability to get married and have a 'less' scope to get divorced as the happier peers are satisfied with their life.' They have adopted a positive attitude of contentment and gratification and hence, they are both physically and psychologically relieved while the other less happy peers have highly advanced fantasies that cause unnecessary stress and anxiety which eventually leads to spoiling their relationships and associations. Thus, they(the lesser happy peers) are 'least' likely to get married and 'more' likely to have a divorce. Therefore, option D is the correct answer.

i would say b. research typically uses a variety of sources