Gall's hypothesis of there being different mental functions localized in different parts of the brain is widely
accepted today.
(A) of there being different mental functions localized in different parts of the brain is widely accepted today
(B) of different mental functions that are localized in different parts of the brain is widely accepted today
(C) that different mental functions are localized in different parts of the brain is widely accepted today
(D) which is that there are different mental functions localized in different parts of the brain is widely
accepted today
(E) which is widely accepted today is that there are different mental functions localized in different parts of
the brain

c) is the correct answer but changes the meaning of original sentence
i find b) more closer to original meaning though it is wordy

My question is Why is it necessary to have a relative clause (Gall's hypothesis that) here?? what is grammatically incorrect with b)