"Which desk is it?"
どれ stands in for a noun similar to これ this one、それ that one、あれ that one there
どの directly modifies a noun such as この this (noun)、その that (noun)、あの that (noun there)
どれですか - Which one is it?
どの机ですか - Which desk is it?
どれ机ですか - Which one the desk is it? - doesn't make sense
Me: I know "this" is これ on its own and この in front of a noun. And "that" is それ/その for stuff near the listener and あれ/あの for far away stuff. And I know that "what" is どれ on its own.
Duolingo: What's "which" (basically "what") in front of a noun?
Me: I don't know.
Duolingo: It's どの.
どれ entirely takes the place of the noun. You can't use どれ + noun. If you want to use a demonstrative ("which" in this case) and the noun both, you have to use どの.
It's the same with あの vs. あれ, その vs. それ, etc. The -の words are used when you're still mentioning the noun, while the -れ words entirely replace the noun.