(A) A recent hypothesis is the theory that stroke victims who have lost the capacity to speak may be able to communicate again through singing.

(B) A recent theory is that if stroke victims who have lost the capacity to speak, they may be able to communicate again through singing.

(C) Recently, a theory is that stroke victims who have lost the capacity to speak may be able to communicate again through singing.

in all A, B, and C is a common error.
the theory states that/ the theory suggests that is correct
the theory is that is incorrect

(D) It has recently been hypothesized that stroke victims who have lost the capacity to speak may be able to communicate again through singing.

(E) It is a recent hypothesis that stroke victims who have lost the capacity to be able to speak may once again communicate through singing.

capacity and to be able is repetitive


OA plz

+1 for D.

(A) A recent hypothesis is the theory that stroke victims who have lost the capacity to speak may be able to communicate again through singing.

(B) A recent theory is that if stroke victims who have lost the capacity to speak, they may be able to communicate again through singing. --Meaning Change(the word hypothesis is removed)
(C) Recently, a theory is that stroke victims who have lost the capacity to speak may be able to communicate again through singing. --Meaning Change(the word hypothesis is removed)
(D) It has recently been hypothesized that stroke victims who have lost the capacity to speak may be able to communicate again through singing. ---correct

(E) It is a recent hypothesis that stroke victims who have lost the capacity to be able to speak may once again communicate through singing.Awkward

A--"hypothesis is the theory" is nonsensical

B--changes "hypothesis" to "theory", which changes the intended meaning of the sentence

C--same meaning problem as B

D--works

E--"capacity to be able to" sounds awesome , but it is redundant (and thus wordy)

OA is D