The principle of cleaning by soap is surface tension. Surface tension is the property of a liquid that causes its surface to behave like a stretched elastic sheet. When soap is added to water, it reduces the surface tension of water and makes it easier for the water to spread and wet surfaces. This helps the soap to penetrate and remove dirt and oil from surfaces.

Option 2: Floatation is not the principle of cleaning by soap. Floatation refers to the ability of an object to float in a liquid, which is unrelated to the cleaning action of soap.

Option 3: Viscosity is also not the principle of cleaning by soap. Viscosity refers to the thickness or stickiness of a liquid, which does not directly affect the cleaning process.

Option 4: Elasticity is not the principle of cleaning by soap either. Elasticity refers to the ability of a material to return to its original shape after being bent or stretched, which is not relevant to the cleaning action of soap.

In summary, soap cleans by reducing the surface tension of water, allowing it to spread and penetrate surfaces more effectively.