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The statement that best describes the Kinetic Molecular Theory is that it suggests particles of matter are always in motion. This theory is fundamental in understanding the behavior of gases and postulates that all particles, whether in solids, liquids, or gases, are in constant motion. The nature of this motion varies depending on the phase of the matter: in gases, for example, particles move freely and rapidly, while in solids, particles vibrate in fixed positions.

In the context of gas behavior, the kinetic molecular theory provides insights into pressure and temperature, indicating that as temperature increases, the kinetic energy of the particles also increases, leading to faster motion. This inherent motion is what drives the diffusion of gases and their ability to fill containers.

The other options do not accurately represent the theory's principles. The suggestion that gas particles are at rest contradicts the foundational premise of the particles being in motion. The claim that particles are only stationary at absolute zero is too narrow, as it refers specifically to a theoretical state that does not apply to the behavior of most matter under normal conditions. The notion that all particles move at the same speed ignores the variability of kinetic energy among particles, which can differ based on conditions such as temperature and pressure.