Hi, West1001,
Great question. The issue here has to do with how
necessary the reliability of people's reports of their thoughts are. Let's revisit the language of the stimulus:
[r]esearchers must be able to rely on the accuracy...
The "must be" is key. "Must" introduces a necessary condition for the success of researchers' efforts, diagrammed thus:
- Info helps researchers Researchers able to rely on accuracy of reports
In the credited response, "only if" introduces the same necessary condition using different language. These paraphrases are a hallmark of Main Point questions. Consider reviewing sufficient and necessary condition indicator words (Logical Reasoning Bible Chapter 6, page 172, 2017 edition). Otherwise, excellent work and please follow up with further questions.