Correct answer: C) Concentration gradient
The process of movement of a particle in a solution from the region of higher concentration to the region of lower concentration across a membrane is called as concentration gradient.
For example: If we drop a few drop of dye in a glass of water, it will diffuse along a concentration gradient, i.e, the region where dye is found in highest concentration to the region where it is in lower concentration.
It will occur until the concentration of dye become uniform in all the direction in the glass of water.