Final answer:
The statement that is NOT part of the kinetic molecular theory is D. KE(average) is proportional to the gas pressure, as the theory states the average kinetic energy is proportional to temperature, not pressure.
Step-by-step explanation:
The kinetic molecular theory has several key postulates regarding the behavior of gas particles, and understanding these can help us determine which of the provided statements is NOT part of the theory. The first statement, A. gas particles travel in straight paths is true according to the theory, until they collide with either another particle or the wall of the container. Statement B. gas particles are in constant motion aligns with the theory, as it asserts that gas particles move rapidly and continuously. Statement C. gas particles have elastic collisions is also accurate, as the theory maintains that collisions do not result in a loss of energy, but rather in an exchange between particles. However, statement D. KE(average) is proportional to the gas pressure is an incorrect representation of the theory. The average kinetic energy (KE) is actually proportional to the gas's temperature, not pressure, as detailed in the kinetic molecular theory. Therefore, D is the correct answer as it is not part of the theory.