The nature of viscosity is

Explanation

Viscosity is a measure of a fluid's resistance to flow, and it arises from the cohesive forces between the molecules of the fluid. These cohesive forces create an internal friction that resists motion when layers of the fluid slide past one another. The incorrect options highlight other forces that may influence fluid behavior but do not specifically address what defines viscosity. Gravitational force affects the weight and pressure of fluids but does not account for the internal resistance. Electromagnetic forces are not directly related to the viscosity of most fluids, which are primarily governed by molecular interactions. Therefore, cohesive forces are the key to understanding viscosity, making Option C the correct choice.