"What book did you write?"
Translation:¿Qué libro escribiste?
I put Cual. I didn't know the difference between cual and que - this is what I found online: Cuál means 'Which' (as in which one?), and is usually the right word to use when followed by 'to be', as in ¿Cuál es la pregunta? Qué is used when immediately followed by the noun, as in ¿Qué pregunta tienes? Cuál es la pregunta.