The correct answer is: B. Gravity
Gravity is the force that attracts objects to each other. It is the force that keeps the atmosphere close to the Earth’s surface. The atmosphere is made up of gases, which are held in place by gravity. The stronger the gravity, the more tightly the atmosphere is held in place. This is why the atmosphere is thicker at sea level than it is at high altitudes.
Radiation is the emission of energy in the form of waves or particles. It is not a force that holds the atmosphere in place.
Cloud cover is the presence of clouds in the sky. It is not a force that holds the atmosphere in place.
Moisture is the presence of water vapor in the air. It is not a force that holds the atmosphere in place.
Pressure is the force exerted by a fluid on a surface. It is not a force that holds the atmosphere in place.