An action in a sentence can be represented in two ways, namely active and passive voice.

The passive voice always uses the past participle form of the main verb irrespective of any tense. Only the auxiliary verbs depend upon the sentence given in active voice.

The correct answer is C) It is sieved as the sentence is in the present tense and the main verb is used in the third form.

Options A, B, and D are not correct as the sentence is not in the present continuous tense, past tense and helping verb is missing respectively.

When milk starts boiling, it is sieved into the cup.