Which
adjective, こそあど word
Remember how どれ means "which one"? どの has a very similar meaning: which. Whereas どれ is used on its own, どの is used before a noun to ask "which" out of three or more options. So for example, どの本 is "which book (out of that pile)?". Notice how question words often start with a ど? We'll be seeing more of these!
どの子がマリアちゃん？
Which child is Maria?
どのホテルにしようかな。
I wonder which hotel I should stay at.
どのタイミングでできますか？
When can you do it?