"Which of these jackets is warm?"
Translation:Laquelle de ces vestes est chaude ?
"Quel" is a masculine adjective, and this sentence calls for a feminine pronoun, so it's "laquelle". That's because the interrogative "laquelle" is all on its own at the start of the sentence, behaving like a noun. You could rearrange it so that "quelle" would work: "Quelle veste est chaude ?"; here the interrogative is directly modifying the noun "veste" so we use an adjective. Duo doesn't accept this answer because that would mean changing the sentence. To use "quel" you would also have to replace "veste" with a masculine noun, e.g. "manteau".
For anyone wondering why it's "laquelle" and "chaude" instead of "lesquelles" and "chaudes", it's because those words are only referencing one of the many jackets that are being referred to. You can see it happen in the English sentence too, where the verb "to be" is conjugated as "is" instead of "are" since only one of the jackets is being spoken about by the verb