"Des pommes ? Lesquelles ?"
- Singular form : masculine / feminine Lequel / laquelle = which one
- Plural form : masculine / feminine Lesquels / lesquelles = which ones
- Singular form : masculine / feminine Quel / Quelle = which
- Plural form : masculine / feminine Quels / Quelles = which
Ex : "Lequel veux-tu ?" = "Which one do you want ?" - "Je ne sais pas dans quel panier le mettre." = "I don't know in which basket I should put it."
Also, note that in an exclamation or interrogative sentence, "Quel / Quels / Quelle / Quelles" can be translated as "What / What a".
Ex : "Quelle magnifique journée !" = "What a beautiful day !" - "Quel est ce bruit ?" = "What is that noise ?"