When gas is compressed then molecules of gas will come closer to each other. Though molecules already had kinetic energy and on compressing the molecules will collide more rapidly with each other.
As a result, energy will be released in the form of heat. Thus, there will be rise in temperature.
Hence, we can conclude that when a gas is compressed, the molecules are pushed closer together causing the gas to rise in temperature.
The answer is "rise in temperature". Think of when you're in a crowded place. When it's crowded it gets hot because of the energy radiating off of everyone around you.
