The working principle of a washing machine is centrifugation. This process involves the fast rotation of clothes within the washing machine to separate water from them by means of centrifugal force.

Option-2, Dialysis, is a process used in medical science to remove waste and excess water from the blood. It is used mainly for people who have kidney disease. So, this is not related to a washing machine.

Option-3, Reverse osmosis, is a process often used in water purification systems to remove impurities. It forces water through a semi-permeable membrane to remove ions, molecules, and larger particles. Again, this is not related to a washing machine`s function.

Option-4, Diffusion, refers to the process of particles spreading from an area of high concentration to an area of low concentration. While diffusion might occur in the washing machine as the detergent spreads through the water, it`s not the primary operating principle.

So, option-1, centrifugation, is the correct answer.