Which of the following is a measure of a fluid's resistance to flow?

Viscosity is defined as a measure of a fluid's resistance to flow. It describes how thick or sticky a fluid is and indicates how easily it can move. For example, honey has a higher viscosity than water, making it flow more slowly. In the context of refrigeration systems, understanding viscosity is crucial because it affects the efficiency of refrigerants within the system. Higher viscosity can lead to increased resistance in pipes and components, impacting flow rates and the overall performance of the refrigeration cycle.

Density refers to the mass of a substance per unit volume, pressure is the force applied per unit area, and temperature is a measure of thermal energy. While all of these properties are important in understanding fluid dynamics and refrigeration operation, they do not specifically address resistance to flow in the same way that viscosity does. Thus, viscosity is the correct choice for identifying a fluid's resistance to flow.