The answer is A: tells
In the sentence The teacher nags students to turn in their homework, the verb nag conveys a negative attitude, in annoying way, almost complaining. The other verbs: bothers (upsets, harasses etc), hassles (worries, troubles etc) and pressures (compels, coerces etc) also manifest sort of an approach frustrated with other previous requests. The verb tell is an adequate substitute for 'nags'.