Alright, you know that kio means what and it’s a kuestion word. You also know that tio means that. There is another kuestion word:
The Esperanto word for where is kie.
It’s easy to remember, if you think that, pronunciation-wise, you could write what as whot - which is kio, and you can write where - which is kie.
I bet you can use the kio - tio analogy to kie - ? and guess the word there. Try to guess and to apply it.