Explanation
Correct answer: A.
Choice A is the best answer. The convention being tested is the use of verb forms within a sentence. The main clause of a sentence requires a finite (tensed) verb, and this choice supplies the finite present tense verb “explain” to indicate what the scientists do.
Why the other choices are wrong
Choice B
Choice B is incorrect because it results in an ungrammatical sentence. The nonfinite participle “having explained” doesn’t supply the main clause with a finite verb.
Choice C
Choice C is incorrect because it results in an ungrammatical sentence. The nonfinite participle “explaining” doesn’t supply the main clause with a finite verb.
Choice D
Choice D is incorrect because it results in an ungrammatical sentence. The nonfinite to-infinitive “to explain” doesn’t supply the main clause with a finite verb.