Final answer:
The correct verb to fill in the blank is 'están'. It indicates that María y Carlos are currently studying in the library.
Step-by-step explanation:
The correct verb to fill in the blank in the sentence is están. María y Carlos están estudiando en la biblioteca. In this sentence, the subject is the plural form of María y Carlos, so the appropriate verb form is also plural, which is están. This verb form indicates that María y Carlos are currently studying in the library.
Learn more about Verb conjugation in Spanish