Sentence Correction

562. 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
(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,

what is the correct answer ? D???

The OA is D. But I am not sure what is wrong with B

D is indeed the correct answer as it correctly uses 'is undoubtedly that' and the construction of the sentence is also not making changes to the original sentence.

B seems to be proposing something 'if yound students....' which is not correct whereas it is given as a fact in the original sentence. Also the construction of the sentence is not good.

Please correct me if I am wrong!

D is the correct answer ! but could any one of u please explain what is wrong in A . And what is the construction or basic thing that makes D a perfect answer ..

Thanks
Senthil

A is incorrect because it uses 'One reason......undoubtedly is because...'. You should be using 'that' with reason and not because...

D is correct simply because

A - One reason is .... because
B - If ... changes the meaning of the sentence from certainty to uncertainly
D - One reason is .... that

No need of one reason and because together.
so B stands out for the single reason.
undoubtedly that is idomatic.

No need of one reason and because together.
so B stands out for the single reason.
undoubtedly that is idomatic.

The rule being tested here is

noun + to be + noun/adjective

A - because ..... is not an adjectival clause
D - That ... is an adjective clause, hence follow the above rule