Question

-¿____está Marta? --Está en el gimnasio. Dónde Cómo Adónde Qué

Answer 1

Donde está Marta?
Esta en el gimnasio

Answer 2

Answer: Donde

Explanation:
Como means how.
Que means what.
The only appropriate answers are either donde or adonde. The reason it's not adonde, is because you use adonde when asking "to where" or "where to" if that makes sense. If you're simply asking where someone is, you say donde.