We never say "wait in a file" or "wait in a row". If we are standing, one person behind another waiting for something (in this case, waiting to reach the counter at the Post Office"), then in BrE, we are waiting "in a queue" and in AmE, we are waiting "in line". Note that the AmE version does not use the indefinite article so could not be correct in your exercise. Of those choices, the only one which can go after "in a" is "queue".
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