Answer : 3. "molecules of the dispersion medium colliding with dispersed phase particles"
Explanation :

Answer: C) molecules of the dispersion medium colliding with dispersed phase particles Explanation: When their is a colloid in the solution, then it will show the Brownian movement, such as dust particle in the air will show the Brownian movement.   Hence, Brownian movement arises due to the impact of the molecules of the dispersion medium with the colloidal particles. It has been postulated that the impact of the molecules of dispersion medium on the colloidal particles are unequal.   This leads to the zig-zag (random) motion of the colloidal particles.