Look at the second half of the sentence for a clue: teach the lesson again and give a second test. This describes a direct corrective action taken to fix a specific problem, the poor grades.
Rectify specifically means to put right a specific fault or error, which matches this corrective action closely. Revise would fit if the sentence talked about updating a lesson or a book, not fixing grades. Reform suits large changes in systems or behavior, not a single test score problem. Right as a verb is grammatically possible but is rarely used in formal or standard test English compared to rectify.
Since the context is about directly fixing a specific problem through corrective steps, rectify is the word that matches best.
The correct answer is option A, Rectify.