Here is my take on the process of getting the correct answer:
One reason why more young people lose their virginity during the summer than at other times of the year undoubtedly is because school vacations give adolescents more free time.
(A) One reason why more young people lose their virginity during the summer than at other times of the year
undoubtedly is because...
here undoubtedly is incorrectly placed before is, the correct meaning should imply: One reason...is undoubtedly...(B) If young people
lose their virginity more during the summer than other seasons, it is undoubtedly because.....
it doesn't make sense to lose their virginity more, the correct meaning should imply more people loose their virginity(C) One undoubtable reason that young people
lose their virginity more during the summer than other times is......
same as (B)(D) One reason more young people lose their virginity during the summer than at other times of the year is undoubtedly that
(E) Young people
lose their virginity more often during the summer undoubtedly because, for one reason,.........
same as (B)