Final answer:
The correct verb form to fill in the blank is 'trabaja' because 'Juan' is singular.
Step-by-step explanation:
The correct verb form to fill in the blank is C. trabaja.
This is because the subject 'Juan' is singular and requires a singular verb form. In Spanish, the third person singular form of the verb 'trabajar' is 'trabaja'.
For example, 'Juan trabaja en una tienda' translates to 'Juan works in a store'.
Learn more about Verb conjugation in Spanish