Explanation
Correct answer: D.
Choice D is the best answer. The convention being tested is end-of-sentence punctuation. This choice correctly uses a period to punctuate a declarative sentence that asks an indirect question ("Researchers Amit Kumar and Nicholas Epley investigated how people perceive acts of kindness").
Why the other choices are wrong
Choice A
Choice A is incorrect. The structure of the sentence requires a declarative clause at the end of the sentence that states what Kumar and Epley did, not an interrogative clause that asks a direct question, such as "how do people perceive acts of kindness."
Choice B
Choice B is incorrect. The structure of the sentence requires a declarative clause at the end of the sentence that states what Kumar and Epley did, not an interrogative clause that asks a direct question, such as "how do people perceive acts of kindness?"
Choice C
Choice C is incorrect. It’s unconventional to use a question mark in this way to punctuate a declarative sentence that asks an indirect question, such as "Researchers…kindness."