Explanation
Correct answer: D.
Choice D is the best answer. Misaka served in World War II before he joined the New York Knicks in 1947. To show that a past occurrence took place before another past occurrence, we need to use “had” + the past tense form of the verb. This is called the past perfect tense.
Why the other choices are wrong
Choice A
Choice A is incorrect. Misaka served in World War II before he joined the Knicks. Both events are in the past, but his service in World War II happened earlier, so we need a verb that makes it clear that his service (and the two national championships) had ended by the time he joined the Knicks.
Choice B
Choice B is incorrect. “Was already serving” forms the continuous past tense, which we use when we’re showing a past action that was ongoing. Misaka served in World War II before he joined the Knicks. Both events are in the past, but they’re not happening at the same time, so we shouldn’t use the continuous past tense here.
Choice C
Choice C is incorrect. Misaka served in World War II in the past, so we shouldn’t use the present tense “serves.”