intentional necro for posterity:
the study simply shows the quantity of relationships, there is nothing to say the DURATION of them. For example, both groups could in theory have been single / partnered for the exact same amount of time, just that the first group had a higher quantity of partners.
so it's impossible to infer if one group was single (or partnered) longer than another, meaning that the likelihood is also indeterminate.
D is actually a poorly worded answer, because they don't qualify "many" and it could just have been 1 person who had all of the relationships in each group (however unlikely that is).
But since it appears that the majority of respondents would consider at least 1 relationship to be "serious" given that the 18 months qualifies all answers as "new relationships" then that is under 2 years and so therefore is correct.