"What book did you write?"
Translation:¿Qué libro escribiste?
I wrote '¿Qué libro tú escribiste?' but it was marked wrong. 'Tú' can be omitted, but it's not wrong to use it, right?
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.
¿Cuál libro? = Which book?
cuál = what only when it comes before a verb. Otherwise cuál + Sustantivo = which + Noun