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

(E) that can be located by the pulsating radio waves
they emit

I go for (E). Subjunctive. What is OA?

A, C, and D are wrong right out of the gate because they all contain "it," which doesn't refer properly to the plural "pulsars."

B is plagued by the wording "to be located," which implies some sort of future inevitability (as in "Smith is to be arraigned next week"). The correct meaning of the sentence is captured by E.

By the way, there's no subjunctive in this problem; you may want to go back and review that.

An example of subjunctive: "For astronomers to locate a pulsar, it is imperative that the pulsar emit radio waves." I know it's weird to use imperatives with an inanimate object, but at least you get the idea.