Which of the following best defines a fluid?

The definition of a fluid encompasses both liquids and gases, making the choice of a liquid or gas the most accurate response. Fluids are characterized by their ability to flow and take the shape of their container, which applies equally to both states of matter.

In contrast to solids, which retain a fixed shape, fluids will easily deform under an applied force. Additionally, while a mixture of gases might represent a specific situation, it does not capture the broader definition that includes all liquids and gases. The option referring to compressible materials only is limiting, as it does not account for incompressible liquids or the wide range of fluids that exist in different states. Thus, defining a fluid as either a liquid or gas effectively encompasses the core characteristics that define fluid behavior in refrigeration and air conditioning systems.