Final answer:
The correct verb form is 'es' (OC).
Step-by-step explanation:
The correct verb form to fill in the blank in the sentence 'Mario muy simpático.' is 'es' (OC). The verb 'es' is the appropriate form of the verb 'ser' (to be) in the third person singular, which is used to describe a characteristic or attribute of someone. In this case, the sentence is saying 'Mario is very nice.'
Learn more about Verb forms in Spanish