Sentence Correction (SC) | Re: Researchers studying the brain scans of volunteers who pondered ethica

The basic tenet of modification is that if a modifier is not separated from its modified word by a comma, then it modifies only the word before. Therefore, A clearly makes out that it is the volunteers who pondered.
Where is the while mentioned in A? I don't think we need to worry about something that is not mentioned in the text. Can't they study the scans, after the volunteers have completed the pondering? The purpose of a scan I feel is just that
It must be appreciated that D is a blatant