Concept:Viscosity is the internal friction that opposes the relative motion between adjacent layers of a fluid.Explanation:When a liquid flows, its different layers move at different speeds.The force that resists this sliding motion between layers is called viscosity.This friction acts like an internal resistance within the liquid.For instance, honey has higher viscosity than water, so it flows more slowly.Thus, the friction between two layers of liquid in relative motion is known as viscosity.Answer:C. Viscosity