Final answer:
The appropriate verb to fill in the blank is 'están'
Step-by-step explanation:
The appropriate verb to fill in the blank in the sentence is C. están.
The sentence is talking about María and Carlos, so the correct verb form is the third-person plural form of the verb 'estar'. In this case, 'están' is the correct form.
For example, a complete sentence would be 'María y Carlos están estudiando en la biblioteca.'
Learn more about Verb conjugation in Spanish