The answer to: ¿En qué piensas, Emily? is: Pienso en viajar a Argentina. This means, "What are you thinking about, Emily?". And the answer is "I'm thinking about traveling to Argentina". The options with the verb "creo" are incorrect because that verb means "I believe" which is not a suitable answer for the question Emily is being asked. And the option that says "pienso que viajar" is grammatically incorrect.
