One reason why more young people lose their virginity during the summe

^ D ^

A and E are redundant as we have both "because" and "reason" within the same sentence. So, they are out.

B is out because it talks as if there is only one reason. It sounds too certain of itself !! We dont have the meaning "one of the reasons" as in the original sentence.

C is out as the sentence construction is not good. After "is" we need "that" !!

you dont need one reason and because at the same time

(A) One reason why more young people lose their virginity during the summer than at other times of the year undoubtedly is because
(B) If young people lose their virginity more during the summer than other seasons, it is undoubtedly because
(C) One undoubtable reason that young people lose their virginity more during the summer than other times is
(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,

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)

I think I have understood why D is correct but just wanted to know- can an adverb modify an entire clause? What is undoubtedly modifying in option D?

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