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Please tell us which one or ones you think are correct and why you believe they are correct. It will help us understand why you are confused.
Also, if this is a multiple choice question, you are required to tell us which answer you think is correct, before we can help you.
yes, it is a multiple choice question and 'everyone' is the option here. My question is why are the others wrong? why can't we use them? One reason given was 'all' and 'all the people' give a vague count (which I did not find convincing) and everyone sounds better that than every person...
All is not specific. It could refer to anything in the building: rats, chairs, cockroaches, or people. It clearly will not work.
All the people is more specific, but you have to specify which people you refer to. If it had been all the people in the building or all the people in the office or all the people on the tour, it would have been fine. When you say that all the people left the building, I think most English speakers will assume you meant everyone in the building, but some people will have doubts. You would be understood, if you said this.
Every person is similar to all the people. Native speakers tend to add a prepositional phrase to explain which people or where they were. Every person is rarely used by itself, though it would be understood, if you chose this option.
In the end, your best choice is everyone, but this is a very difficult question. You would need a very strong understanding of English, to get this right.