どれWhich (of three or more)

どれ is from the same family of words as これ, それ, and あれ. These words are called こそあど言葉(ことば) in Japanese. Unlike the other 3, which all point to a specific thing, words starting with ど are designed for enquiring about something unknown. どれ can roughly be translated as 'which' in English, and is used for enquiring about a group (of 3 or more things).

From these examples, we can see that the particle が is often used with どれ, especially when preceding questions. However, when どれ follows a noun, は will be used.

When there are only two choices to select between, どちら + の, or どっち + の will be used instead of どれ. This is a habit that has become part of the language over time, and was not originally how these words were used.