C .  
Air is not a matter as it cannot be seen.
D .  
Air doesn't exert pressure.
Answer: Option A : A Air occupies space and has mass, therefore, it is matter. Air can be compressed and it exerts pressure in all the directions. For example, when we inflate a balloon, air occupies the space in the balloon and exerts pressure on the walls of the balloon. Air around us is a mixture of gases. It contains nitrogen, oxygen, argon, other gases and also water vapour.
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