Pulsars are generally believed to be fast-spinning neutron

Pulsars are generally believed to be fast-spinning neutron stars to be located by the pulsating radio waves it emits.

A) to be located by the pulsating radio waves it emits
B) to be located by the pulsating radio waves they emit
C) locatable by the pulsating radio waves which it emits
D) emitting pulsating radio waves by which it can be located
E) that can be located by the pulsating radio waves they emit

An easy entry into the question is the most visible error of the singular 'it' to refer to the plural Pulsars. A, C, and D are gone in a stroke.

Between B and E, we must appreciate the ethos of GMAT that whenever we report scientific matters or study reports, the content of the matter is best presented by a relative clause starting with the relative pronoun 'that'. E is in line with the vibe of GMAT