This is the 197th most frequent Japanese word.
どこ
"Where"
Here, 'どこ' is used to inquire about the location of the passport in a situational context.
パスポートがどこにあるかわかりません。
I don't know where my passport is.
In this instance, 'どこ' is used to ask which aspect or section of the store is popular.
あの店がどこが人気なのか教えてください。
Please tell me which part of that store is popular.
In this sentence, 'どこ' (doko) asks about a location where the meeting will be held.