Questions & Answers about Quel est ton film préféré?
In French, quel is an interrogative adjective that must be followed by a noun (or être + noun/adjective). It agrees in gender and number with that noun.
- Quel = which / what (followed by a noun)
- Quel film = which/what film
- Quoi = what (used mostly after prepositions or at the end of a question)
In Quel est ton film préféré ?, quel is used because it refers to film, a masculine singular noun (implied in the structure quel est [noun]). Using quoi here would be ungrammatical.