There was a question in one of the quizzes I took and it led me to this grammar topic (questions with qué):
Which flowers do you prefer?
¿ _____ flores prefieres ?
I put cuales in the blank, but was marked wrong, it said the answer was qué.
I went to the lesson on cual/cuales (interrogative pronouns) and there was a sample sentence referring to flowers, using cuales prefieres.
In the English translation to the quiz it uses which and I assumed that a select few flowers were being referred to, thus the answer should be cuales. It is a bit ambiguous.