Answer:

The behavior of a perfect gas is governed by the pressure exerted by the gas, the volume occupied by the gas, and the temperature of the gas. All three of these variables play a crucial role in determining the physical properties of a perfect gas.

Pressure:
Pressure is defined as the force exerted by the gas per unit area. In a perfect gas, the pressure is directly proportional to the number of gas molecules and their average kinetic energy. As the number of gas molecules or their kinetic energy increases, the pressure of the gas also increases. Similarly, as the number of gas molecules or their kinetic energy decreases, the pressure of the gas decreases. Therefore, changes in pressure can significantly impact the behavior of a perfect gas.

Volume:
Volume is the amount of space occupied by the gas. In a perfect gas, the volume is directly proportional to the number of gas molecules. When the volume of a gas increases, the gas molecules have more space to move around, resulting in a decrease in the frequency of collisions between molecules. This leads to a decrease in the pressure of the gas. On the other hand, when the volume of a gas decreases, the gas molecules are confined to a smaller space, increasing the frequency of collisions and thus increasing the pressure. Therefore, changes in volume can affect the behavior of a perfect gas.

Temperature:
Temperature is a measure of the average kinetic energy of the gas molecules. In a perfect gas, the temperature is directly proportional to the average kinetic energy of the gas molecules. When the temperature of a gas increases, the average kinetic energy of the gas molecules increases, leading to an increase in the pressure of the gas. Conversely, when the temperature of a gas decreases, the average kinetic energy of the gas molecules decreases, resulting in a decrease in the pressure. Therefore, changes in temperature can also influence the behavior of a perfect gas.

Conclusion:
In summary, the behavior of a perfect gas is governed by the pressure exerted by the gas, the volume occupied by the gas, and the temperature of the gas. Changes in any of these variables can significantly impact the physical properties of a perfect gas. Therefore, the correct answer is option 'D' - all of these variables are important in understanding the behavior of a perfect gas.