The pressure of an enclosed gas depends on: the altitude about sea level all of the above the number of atoms per molecule its chemical composition the number of molecules in a unit volume and their average kinetic energy

The pressure of an enclosed gas primarily depends on the number of molecules in a unit volume and their average kinetic energy. This is because, according to the kinetic molecular theory, gas pressure is caused by collisions of gas molecules with the walls of the container. The more molecules there are in a given volume and the higher their average kinetic energy (which is related to temperature), the greater the pressure.<br /><br />While altitude above sea level can affect atmospheric pressure, it does not directly affect the pressure of an enclosed gas unless the container is flexible and allows for volume changes. The number of atoms per molecule and its chemical composition can influence other properties of the gas but do not directly determine the pressure in the context of an ideal gas. Therefore, the correct answer is:<br /><br />- The number of molecules in a unit volume and their average kinetic energy.