どの
どの · dono · JLPT N5 · suffix
which
do + no: ど marks a question (どれ, どこ), の links it to a noun
Part of the ko-so-a-do set: この this, その that, あの that over there, どの which. The ど- start always signals a question word, and the の ending means a noun must follow: どの本 = which book.