According to the kinetic theory of gases, gas particles are in constant, random motion and collide with the walls of their container. Each collision imparts a small amount of momentum to the wall.

According to the kinetic theory of gases, gas particles are in constant, random motion and collide with the walls of their container. Each collision imparts a small amount of momentum to the wall.

• Pressure is defined as force per unit area ($P = F/A$).
• Force is defined as the rate of change of momentum ($F = \frac{\Delta p}{\Delta t}$).

Combining these definitions, the pressure exerted by a gas on the walls is the rate of change of momentum imparted to the walls per unit area.

Let's evaluate the options:

• (A) "Momentum imparted to walls per unit area" is incorrect because it doesn't include the "rate" aspect (i.e., per unit time).
• (B) "Change in momentum per unit volume" is incorrect. Pressure is force per area, not momentum per volume.
• (C) "Rate of change of momentum imparted to the walls per second per unit area" accurately describes pressure. "Rate of change of momentum" is force, and "per unit area" makes it pressure. The "per second" reinforces the "rate" aspect.
• (D) "Change in momentum imparted to the walls per unit area" is incorrect for the same reason as (A); it lacks the "rate" aspect.

The final answer is $\boxed{\text{C) Rate of change of momentum imparted to the walls per second per unit area}}$.