Quasars, at billions of light-years from Earth the most dist

Quasars, at billions of light-years from Earth the most distant observable objects in the universe, believed to be the cores of galaxies in an early stage of
development.

(A) believed to be
(B) are believed to be
(C) some believe them to be
(D) some believe they are
(E) it is believed that they are

Please explain strategically