"Which of those nine people was it?"
Translation:Který z těch devíti lidí to byl?
Disclaimer: I am learning, too, so one of the CZ natives on the team may correct my answer...
My guess is that to is not accepted as the last word in the sentence because it is one of many small words that like to be in the "second position" of the sentence or clause. The "second position" is not just after the first word; it is also after the first "unit of meaning." Here, the first unit of meaning is "Který z těch devíti lidí," so to, following it, is in the second position.