Explanation

Reading the question: we have an argument, a prediction about the future based on how things have gone after this switch to more experienced salespeople. The question stem asks for what "would be most useful to know"; we will prove by stronger terms and find an assumption central to the argument. Nothing is more useful to know in evaluating an argument than whether its key assumption is correct. A couple of assumptions do come to mind: maybe more experience salespeople will improve their performance faster than the less experienced staff would have. Maybe they will turn over less often. Maybe, though they are more expensive now, their salaries will rise less slowly. So we're expecting an assumption that involves "more experienced improving faster," in terms of performance, or salary, or something else.

Applying the filter: Only (D) matches our filter. Choice (A) does not differentiate between more and less experienced salespeople, so it's immaterial to the prediction the argument is making. (B) and (E) don't address why things might be any better in the future than they have been this past year under the more experienced team. Choice (C) is an irrelevant comparison.

Logical proof: the word "whether" is a clue that we can establish logical proof through analysis by cases. We can consider two extremes. If (D) is true, the company could, for example, simply not train the new salespeople at all when hiring experienced salespeople; if turnover is large, that could save a lot of money and improve the outcome. And if (D) is not true, it's a further reason to agree with the prediction in the argument. The correct answer is (D).