Consider the diagram. mc006-1.jpg which postulate of the kinetic-molecular theory best describes the event in the diagram? most of the volume of a gas is empty space. all collisions between particles are perfectly elastic. there is no force of attraction or repulsion between gas particles. the average kinetic energy of particles depends only on temperature.

I found the exercise on the internet. Attached is a pic of the diagram.
Correct
"All collisions between particles are perfectly elastic." 
This is called elastic collision. Elastic collision is the encounter between two molecules where the total kinetic energy of the molecules before the encounter is equal to their total kinetic energy after. The perfectly elastic collision it only happens, though, when the kinetic energy is not transformed into other forms of energy.