Final answer:
The correct verb to fill in the blank is 'están' (option A).
Step-by-step explanation:
The correct verb to fill in the blank in the sentence is están (option A).
The sentence is saying 'María and Carlos are studying in the library.'
In this case, 'María y Carlos' is the subject of the sentence and it is plural, so we need a plural verb. The correct form of the verb 'estar' (to be) in the third person plural is 'están'.
Learn more about Spanish verb conjugation