Final answer:
The correct verb form is vas.
Step-by-step explanation:
The correct verb form to fill in the blank in the dialogue is vas. The question is asking where Marta is going, so we need the second person singular form of the verb ir (to go), which is vas. The sentence would be: Marta, ¿adónde vas ahora? a la clase de arte.
Learn more about Verb forms in Spanish