OF A TYPE vs IS A TYPE

Have a look at this sentence-


Astronomers at the Palomar Observatory have discovered a distant supernova explosion, one that they
believe is a type previously unknown to science.
(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

The correct answer is E.
The correct usage here is ..they believe to be of a type previously unknown to science.