Which gas is filled in chips packets?

Explanation

Foods containing fat/oil when left for a long period of time, become oxidized. When fats and oils become oxidized, they become rancid and their taste and smell changes. Nitrogen gas prevents chips from oxidizing and thus keeps it fresh for a longer time.
Alternative explanation: Avoid oxygen in pure \(O_2\) form or in atmospheric air.  Methane is flammable.  Nitrogen is cheaper than \(CO_2\)