The word rectify means to correct or fix something that is wrong. The sentence talks about poor test grades, which is a problem that needs to be corrected. Rectify fits this meaning perfectly.
Option B, right, can work as a verb meaning to correct, but it is less commonly used in this context and sounds unnatural when talking about grades. Rectify is the standard word used for correcting problems like poor performance.
Option C, revise, means to look over and improve something, usually written work or a plan. We revise a lesson or a document, but we do not revise poor grades themselves.
Option D, reform, means to improve a system, organization or a person's behavior over a long period. It does not fit the immediate action of fixing test grades through re-teaching.
Since the sentence needs a word meaning to correct, the best fit is option A, Rectify.