Quel livre lis-tu ?
English: Which book are you reading?
Interrogative adjectives in French are words used to ask questions about a noun. They agree in gender and number with the noun they describe.
Use interrogative adjectives in French when you want to ask 'which' or 'what' before a noun. They are always placed directly before the noun they refer to.
Quel livre lis-tu ?
English: Which book are you reading?
Quelle couleur préfères-tu ?
English: What color do you prefer?
Quels films as-tu vus ?
English: Which movies have you seen?
Quelles chaussures veux-tu acheter ?
English: Which shoes do you want to buy?