Answer: A
Correct answer: write
After make in the active voice, the verb that follows takes the bare infinitive - no to. So made the students write. The verbs that behave this way form a small group: make, let, bid, see, hear, watch, feel and help, along with the modal auxiliaries. Hence let him go, I saw her leave, he helped me carry it. The curious exception is that make takes the full infinitive in the passive: the students were made to write the essay again. The to reappears when the sentence is turned round, which is why both forms are heard and why the two are confused. Writing fails because a gerund cannot follow make in this construction, and written would make the students the thing written rather than the writers.