The right option is; The molecular collisions are perfectly elastic.
The kinetic molecular theory is a theory that describes the experimental observations about the behavior of gases. This theory is based on five postulates, or assumptions. The idea that no energy is lost when gas molecules hit the walls of a container or each other is explained by the fourth statement (postulate) which states that molecular collision (between gas molecules or with the walls of the container) is perfectly elastic and it does not involve the loss of any energy .