Final answer:
Lowering the temperature or increasing the pressure of a gas without adding heat leads to a decrease in the gas's average kinetic energy, hence the kinetic energy decreases.
Step-by-step explanation:
When the temperature of a gas is decreased, the average kinetic energy of the gas particles decreases, which makes the gas cooler. Conversely, when the pressure is increased sufficiently, it can also lead to a decrease in kinetic energy if the volume of the gas remains constant or decreases, as work is being done on the gas. In the case of decreasing temperature or increasing pressure without adding heat, the answer is (a) Kinetic energy decreases. This is understood through the kinetic molecular theory of gases which correlates the temperature of a gas with the kinetic energy of its particles; as temperature goes down, kinetic energy goes down as well, leading to less forceful collisions of particles with the walls of the container. Therefore, reducing temperature or increasing pressure would lower the kinetic energy of gas particles unless energy is added to the system.