Answer:
The correct answer option is a) collisions between the particles and surrounding molecules.
Step-by-step explanation:
The collisions between the particles and surrounding molecules causes the Brownian motion of particles in a colloid.
Brownian motion is the irregular movement of the microscopic particles in a fluid which bombard into each other.
It basically is the result of the molecules of a dispersion medium colliding with the dispersed particles of the phase.