哪些 (nǎ xiē) asks 'which ones?', used when you expect a plural answer. It contrasts with 哪 (nǎ), which needs a measure word after it (e.g., 哪个人), and 哪个 (nǎ ge), which asks 'which one' (singular). Be careful not to confuse it with 那些 (nà xiē, 'those'), which has a different tone.