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QUICK ANSWER
"Cual" is a form of "cual", a pronoun which is often translated as "which". "De qué" is a phrase which is often translated as "what". Learn more about the difference between "de qué" and "cual" below.
de qué()
A phrase is a group of words commonly used together (e.g., once upon a time).
cual()
A pronoun is a word that stands in for a noun (e.g., she).
1. (relative)