Atheory in science is
a. a synthesis of well-tested ideas.
b. an agreement between trained investigators.
c. an educated guess.
d. forever true.

It would be (a)

Explanation:

A theory is something that can be proved through rigorous and repeated experimentation, while choice (b) sounds tempting i believe it fails at the fact that it says that a theory is an "agreement", sure an agreement could be reached by while trained scientists but that does not necessarily mean that what the scientists agree upon is rigorously tested and repeatable, and so therefore cannot be assumed

That is what I think

Hope this helps :)