哪

哪 (nǎ) is an interrogative pronoun used to ask "which" when there are multiple options. It differs from 什么 (what) in that 哪 implies the speaker already has several possibilities in mind.

The most important point is that 哪 almost always appears together with a measure word immediately following it, e.g. 哪个人 (which person), 哪本书 (which book). When asking about location, 哪儿 (nǎr) or 哪里 (nǎli) is used, meaning "where."