Put the verb in brackets into the Present Continuous form. They (clean) their teeth now.

The sentence requires us to put the verb "clean" into the Present Continuous tense. The Present Continuous (or Present Progressive) tense is used to describe actions happening at the moment of speaking or around the present time. Its structure is: `Subject + is/am/are + verb-ing` Since the subject is "They" (plural), we use "are" as the auxiliary verb. Therefore, the correct form is: "They are cleaning their teeth now." So, the correct answer is "are cleaning". **Answer:** are cleaning