Even though bats prefer very sweet nectar, the animals that attract them have evolved to produce

Choice A is the best answer. The convention being tested is the use of finite and nonfinite verb forms within a sentence. A main clause requires a finite verb to perform the action of the subject (in this case, “a recent study”), and this choice supplies the finite present tense verb “explains” to indie that the study explains why animals that attract bats have evolved to produce moderately sweet nectar. 

Choice B is incorrect because the nonfinite participle “explaining” doesn’t supply the main clause with a finite verb. Choice C is incorrect because the nonfinite participle “having explained” doesn’t supply the main clause with a finite verb. Choice D is incorrect because the nonfinite to-infinitive “to explain” doesn’t supply the main clause with a finite verb.