Explanation
Correct answer: D.
Choice D is the best answer. This sentence is a statement: “Moskowitz was hired by a soda company to determine how much artificial sweetener most people prefer in a diet drink.” So a period is the most appropriate punctuation mark.
Why the other choices are wrong
Choice A
Choice A is incorrect. This doesn’t complete the text in a way that conforms to the conventions of Standard English. This sentence is not a question—it’s a statement. So a question mark is not the appropriate punctuation.
Choice B
Choice B is incorrect. This doesn’t complete the text in a way that conforms to the conventions of Standard English. We already have the verbs “was hired…to determine” in this sentence. The verb “do” is not needed and results in a confusing, ungrammatical sentence.
Choice C
Choice C is incorrect. This doesn’t complete the text in a way that conforms to the conventions of Standard English. This sentence is not a question—it’s a statement. So a question mark is not the appropriate punctuation.