ใด//dai is a literary version of ‘which’ as a question mark.
คนไหน//khon năi vs. คนใด//khon dai
Credit pic: here
ที่ไหน//thêe năi vs. ที่ใด//thêe dai
Which place, where
Warning: As mentioned that this is for literary use, you will sound quite odd if ใด//dai is used instead of ไหน//năi. This is probably found in exam questions, such as:
ข้อใดถูก//khôr dai thòok
Which number (or answer) is correct, A B C or D?