The Interrogative Adjective.

In English this would be "which" + noun.
Which house did they buy?
Which conclusion did you come to?
Which men fought at that battle?

Since in French adjectives (whatever kind) must agree in gender and number with the noun
or pronoun they modify, (see: Adjectives) there are more than one form--and they are:

quel for masculine singular
quels for masculine plural
quelle for feminine singular
quelles for feminine plural.

Examples:

Quel mois sommes-nous. Which month are we in? -- masc.sing.

Quelle maison voulez-vous achetée? Which hous do you want to buy? -- feminine singular.

Quels amis vont avec nous? Which friends are going with us? -- masculine plural

Quelles filles veulent danser? - Which girls want to dance? -- feminine plural.

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Quel and all its forms may also (and is often) used as an exclamation:

Quelle belle jupe!! What a beautiful skirt!!!
Quelles belles roses! What beaufiful roses!
Quels temps! What a weather!