Final answer:
The correct verb form to fill in the blank in the sentence is 'trabaja.'
Step-by-step explanation:
The correct verb form to fill in the blank in the sentence is trabaja. The sentence is talking about Juan, who is a singular subject, so we use the third-person singular form of the verb 'trabajar,' which is 'trabaja.' The correct sentence is 'Juan trabaja en una tienda.'
Learn more about Spanish verb conjugation