Quel/quelle = masculine/feminine singular
quels/quelles = masculine/feminine plural.
Quel auteur tu aimes le plus? -- Which author do you like the most?
Quelle fille t'a apporté ton manteau? -- Which girl brought you your coat?
Quels auteurs tu aimes le plus? -- Which authors do you like the most?
Quelles filles t'ont apporté ton manteau? -- Which girls brought you your coat?
