Final answer:
The driving force behind diffusion is the kinetic energy of molecules, which causes them to move from an area of higher concentration to an area of lower concentration until equilibrium is reached.
Step-by-step explanation:
The principal force driving movement in diffusion is the kinetic energy of molecules. Diffusion is defined as the movement of particles from an area of higher concentration to an area of lower concentration due to the random motion of the molecules. While factors like the concentration gradient and electrical gradient can affect the rate and direction of diffusion, it is the inherent kinetic energy of the molecules that causes them to move and spread out until equilibrium is reached.