The real answer is #2 and here's why -- the piston coming down creates a lower-pressure area in the chamber and the relatively higher pressure air from the intake *flows*E into the lower pressure area. The lower pressure area is called "suction". Technically, atmospheric pressure is pushing the air in, but that definition is already included in the definition of suction, and there is also the lower pressure area in the chamber sucking the air in.
Even for supercharged cars, I don't even believe #3 to be correct because the pressure is no longer atmospheric, but compressed atmospheric.
Cheers,
-Neil.
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