Final answer:
The verb 'saben' completes the sentence as 'Ellos no saben al profesor de música.'
Step-by-step explanation:
The verb that completes the sentence is 'saben'. So the sentence would be 'Ellos no saben al profesor de música.' The verb 'saben' means 'they know' in English. In this context, it means that they do not know the music teacher.
Learn more about verb conjugation in Spanish