In 1990’s, there are more babies born by women over thirty years old than under it.
(A) than under it (the best option)
Comparison is between two age categories. Over thirty than under thirty. It conveys the message.
(B) than were they under it (incorrect: comparison and pronoun error)
Pronoun they giving unclear meanings as they can be babies or women. Moreover, the comparison should be between two ages women not clause.
(C) than had been under it (Tenses and comparison error)
Had been is wrong as the main clause is in present and there is a clause comparison error.
(D) than were the babies (incorrect: comparison and tenses error)
Babies is in comparison with thirty years which is incorrect and the main clause is in the present hence rest should be in the present.
(E) than those were under it (incorrect: pronoun and comparison error)
The same error as options D and E.