Final answer:
The correct verb is 'están'. It is the third-person plural form of 'estar' meaning 'to be' in Spanish, used to express location or state.
Step-by-step explanation:
The correct verb to complete the sentence is 'están'.
So the complete sentence would be 'Los libros están en la librería.'
Here, 'están' is the third-person plural form of the verb 'estar', which means 'to be' in Spanish. It is used to express location or the state of something.
Learn more about Verb conjugation in Spanish