(A) that they believe is
(B) that they believe it to be
(C) they believe that it is of
(D) they believe that is
(E) they believe to be of
Solution:
Idiom
Believe + to be is the correct usage. Hence, A and D can be eliminated.
B and C can be eliminated for the unnecessary use of it.
E is the best choice.
Sentence Correction - 1000 Questions with Solution: