Imagine you have just entered a room and saw your friends talking about something. One of you friends is finishing her sentence (your friends are talking about aircraft, but you don't know that):
They all go to boneyard and stay there for good after they become too old.
You really want to know who "they" in this sentence are. From "stay at boneyard for good" you infer that the subject is inanimate things, rather than humans or animals.
So how would you ask of your friends a question that will help you find out the subject of their conversation?
1) Who goes to boneyard after it becomes too old?
2) What goes to boneyard after it becomes too old?
3) Who goes to boneyard after they become too old?
4) Who go to boneyard after they become too old?
5) What goes to boneyard after they become too old?
6) What go to boneyard after they become too old?
7) [something else?]
(I understand that I can simply ask "What are you guys talking about?", but I want to respond to the last sentence spoken by my friend)