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<u>Answer:</u>
The correct answer option is a) collisions between the particles and surrounding molecules.
<u>Explanation:</u>
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.
Answer is: Not all mixtures have solutes and solvents.
Solution is homogeneous mixture composed of two or more substances.
In aqueous solution, solvent is water and solute (for example salt) is a substance dissolved in water.
Homogeneous solution of salt and water can be separated with heating (evaporating the water).