The kinetic viscosity of a liquid refers to its resistance to flow or internal friction. It is measured by the ratio of the coefficient of viscosity to the density of the liquid.

Option 1 states that the kinetic viscosity is the ratio of the coefficient of viscosity to the density. This option is correct. The coefficient of viscosity represents the internal resistance to flow, while the density refers to the mass or concentration of the liquid. Dividing the viscosity by the density gives us the measure of kinetic viscosity.

Option 2 suggests that the kinetic viscosity is the ratio of the coefficient of viscosity to the surface tension. This is incorrect. Surface tension is the force that acts on the surface of a liquid and is unrelated to the measure of kinetic viscosity.

Option 3 states that the kinetic viscosity is the ratio of density to the coefficient of viscosity. This is incorrect. The correct ratio is the other way around, with the coefficient of viscosity being divided by the density.

Option 4 suggests that the kinetic viscosity is the ratio of surface tension to the coefficient of viscosity. This is incorrect, as surface tension is not involved in the calculation of kinetic viscosity.

Based on the above explanations, option 1 is the correct answer.