GMAT Sentence Correction (SC) | Re: According to a study by the Carnegie Foundation for the

According to a study by the Carnegie Foundation for the Advancement of Teaching, companies in the United States are providing job training and general education for nearly eight million people, aboutequivalent to the enrollment of the nation’s four-year colleges and universities.
(A) equivalent to the enrollment of "Equivalent" is an adjective, modifying the amount (which is another red flag). The "number" must not be omitted. None of them follow so it's grammatically incorrect.