Correct form: "
Not only X
but also Y"
(A) It was believed that a thorough knowledge of Latin would
not only enable students to read the classics,
also enabling them to think clearly and precisely.
"Not only X also Y" is incorrect
Also would enabling them is not correct
(B) It had been believed that a thorough knowledge of Latin would
not only enable students to read the classics
but rather enable them to think clearly and precisely.
"Not only X but rather Y" is incorrect
(C) It was believed that a thorough knowledge of Latin would not only
enable students to read the classics but also
enabling them to think clearly and precisely.
"Not only X but also Y" form; however, enable is not parallel to enabling
(D) It used to be believed that a thorough knowledge of Latin would enable a student to be able to read the classics
but also enable them to think clearly and precisely.
missing "not only"
(E) It was believed that a thorough knowledge of Latin would enable students not only to read the classics, but also to think clearly and precisely.
"Not only X but also Y". Correct. Also, when the stem can be connected logically to both X and Y.
E is the correct answer