What happens when the air between the rubber sucker and plane surface escapes out by pressing it firmly?
1. Stick to the surface
2. Does not stick
3. Stick loosely
4. None of the above

Hint: When air is being escaped out between the rubber sucker and plane surface, a vacuum is created. In the vacuum air pressure is negligible which allows external air pressure to dominate.

Complete step-by-step answer:

Atmospheric pressure – Atmosphere is made up of gaseous molecules, force per unit area exerted by a column of this atmosphere is called atmospheric pressure.
When a rubber sucker is pressed onto a plane surface, air escapes out between the rubber sucker and plane surface. Now negligible air left inside the rubber sucker, shown in the above figure.
A negligible air pressure is created on the inside surface of the rubber. Thus, the external atmospheric pressure dominates. Due to which, the rubber sucker can’t be pulled out easily.
Thus, when the air between the rubber sucker and plane surface escapes out by pressing it firmly, rubber suckers stick to the surface.
So, option A is correct.

Additional information:
Pressure gradient plays an important role in flowing air. Air always flows from high pressure to low pressure. We a part of the earth surface is heated more than the other, its gaseous molecules expand. Thus, low pressure is created there and this part acts as a suction area for the wind coming from the high pressure area.

Note: Mind the fact that the air in this question is let out slowly by pressing the rubber sucker against the surface. Doing this avoids the situation that if a small opening is created between the rubber sucker and the surface, which allows air to get inside, the pressure inside increases to become equal to the atmospheric pressure. Thus, losing the force to stick to the surface and the rubber sucker comes off.