The correct answer is option 3, gravitational pull. Atmospheric pressure is the force per unit area exerted on a surface by the weight of air above it. This pressure is caused by the gravitational force pulling the air molecules towards the surface of the Earth. The greater the mass of air above a particular area, the greater the atmospheric pressure will be. This gravitational pull is what keeps the atmosphere close to the Earth`s surface and prevents it from dispersing into space.
Options 1 and 2, rotation and revolution of the Earth, do not directly cause atmospheric pressure. While these motions can affect atmospheric circulation patterns and weather systems, they do not directly contribute to the pressure exerted by the atmosphere.
Option 4, the uneven heating of the Earth, does indirectly contribute to atmospheric pressure. Unequal heating of the Earth`s surface leads to differences in temperature and density, which in turn affect air pressure distribution. However, the primary cause of atmospheric pressure is still the gravitational pull.
Therefore, the correct answer remains option 3, gravitational pull.