It is useless _ them; they are sure to have left the house by now

Explanation

The correct answer is 'to call' because the sentence discusses a potential action the speaker is considering taking in the present. The infinitive 'to call' expresses this idea of contemplation or planning without execution. The other options do not fit the context: 'call' suggests a command, 'called' implies the action has already happened, and 'calling' indicates an ongoing action. None of these align with the sentence's suggestion that the action is merely being contemplated.