Final answer:
The appropriate verb form to fill in the blank is 'trabaja'.
Step-by-step explanation:
The appropriate verb form to fill in the blank in the sentence is option C. trabaja.
When talking about Juan, who is a singular third person, the correct form of the verb is 'trabaja'.
Example: Juan trabaja en una tienda. (Juan works in a store.)
Learn more about Verb conjugation in Spanish