Explanation
Correct answer: A.
Choice A is the best answer. The convention being tested is subject-modifier placement. This choice makes the noun phrase "the researchers" the subject of the sentence and places it immediately after the modifying phrase "knowing…day." In doing so, this choice clearly establishes that the researchers—and not another noun in the sentence—know that flying insects keep their backs pointed toward sunlight during the day.
Why the other choices are wrong
Choice B
Choice B is incorrect because it results in a dangling modifier. The placement of the noun phrase "the researchers’ theory" immediately after the modifying phrase illogically suggests that the researchers’ theory knows that flying insects keep their backs pointed toward sunlight during the day.
Choice C
Choice C is incorrect because it results in a dangling modifier. The placement of the noun "moths" immediately after the modifying phrase illogically suggests that moths know that flying insects keep their backs pointed toward sunlight during the day.
Choice D
Choice D is incorrect because it results in a dangling modifier. The placement of the noun "moths" immediately after the modifying phrase illogically suggests that moths know that flying insects keep their backs pointed toward sunlight during the day.