GMAT Sentence Correction (SC) | Re: A group of sudents who have begun to clean up Frederick Law

The solution has been properly explained, but someone earlier mentioned how "have" was there instead of "has" indicating plurality.

The subject of the sentence is "A group," which means the sentence must be correct with all descriptive phrases cut out. "A group...believes that..." is clearly correct. That leaves the descriptive phrase to be selfcontained, however. In "students who have," the "have" is describing thestudents who make up the group