Answer:
Limpiar
Step-by-step explanation:
The correct answer is 'limpiar' because it is the only one that fits the rest of the sentence. It means 'the students have to clean the whole school'.
The other verbs ('llorar', 'pasar', 'contestar') do not have anything to do with the context of the sentences. The verbs mean: cry, pass, answer.
The sentence of the exercise is in the present tense and the verb is in the infinitive form.