A pre-nominal interrogative meaning 'which' or 'what (kind of)', used before a noun (which X?).
どの本が一番面白かった？
Which book was the most interesting?
どの道を行けば駅に着きますか？
Which road should I take to get to the station?
どの学生が代表に選ばれたのか知っていますか？
Do you know which student was chosen as the representative?