Subject verb agreement -- in number e-gmat question doubt

I faced the following question in the assessment test of e-gmat and I am unable to understand why I am wrong?

Question : A May study in Nature found that 65 percent of autistic children share a set of mutations that regulates genes known to influence communication among brain cells.

A) Regulates

B) Regulate



I thought it should be mutations that regulate but my answer was wrong and correct answer is regulates .

Please help