Why does sand not diffuse into the water?

Diffusion is the intermixing of particles of two matter and basically proves that the particles are moving from the region of high concentration to the region of low concentration.

Now, for this, the size of the particles also plays a role. Sand particles cannot be broken down into small particles that are capable to fit into the spaces between the particles of water. Thus, Sand does not diffuse into water.

Hence, the correct option is (B).