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
believe X to be Y is correct idiom.
Between B & E. E is concise and clear. B changes the meaning by saying that DSN is a type itself.