"The girls listen."
Translation:Die Mädchen hören zu.
Many German verbs are separable-prefix verbs. The infinitive is zuhören, but whenever the verb is conjugated or in a main clause, the prefix is separated from the verb. This hasn't been introduced in Duolingo yet... But it will be soon. Separable-prefix verbs are definitely a headache for German learners!
The only reason I knew I had to fill in that blank with "zu" is because of the "What Was That?" story where Martin öffnet die Tür und hört zu. Otherwise I would have absolutely no idea what to put here. I don't know why it's right, either. From other comments, it'll show up in a lesson eventually maybe.