"Where do I check in?"
Translation:탑승 수속은 어디 합니까?
There's two reasons I can think of:
Unlike English, Korean places question words exactly where the answer word would go. E.g. A possible full answer to this exercise is 탑승 수속은 저기 합니다. Notice that 저기 is located exactly where 어디 is in the question. It's like English questions when words aren't rearranged, e.g., "I check in where?" instead of "where do I do check in?". You'll notice this pattern with all the question words in Korean too.
Generally speaking whatever comes before the topic particle is roughly the topic. This allows complex topics like 어제 만난 친구는... ("regarding the friend I met yesterday..."). But I don't believe a topic like 어디 탑승 수속 makes sense (I don't even know how to translate that in English).