What is the definition of a fluid?

The definition of a fluid is a substance that can deform and flow in response to applied forces, which is precisely what option C describes. Fluids encompass both liquids and gases, and their defining characteristic is the ability to take the shape of their container, responding to pressure changes without maintaining a fixed shape. This property of flowing allows fluids to exert pressure in all directions, which is crucial for various applications in HVAC systems and refrigeration.

In contrast, a solid substance that does not deform is characterized by maintaining a fixed shape under normal conditions, which does not fit the definition of a fluid. Similarly, a substance that maintains a constant shape does not exhibit the flowing characteristics of a fluid, and defining a gas as something that cannot change form is misleading, as gases can expand and compress, adapting to different volumes and pressures. Therefore, the correct understanding of fluids aligns with their fundamental behavior of being able to flow and deform when subjected to forces.