Which of the following best defines a fluid?

A fluid is best defined as a substance that can flow and take the shape of its container. This definition encompasses both liquids and gases, which are the two primary states of matter classified as fluids. Unlike solids, which have a definite shape and volume, fluids do not have a fixed shape; they adapt to the constraints of their environment.

This property of fluids is critical in various applications across mechanical engineering, such as in fluid dynamics, hydraulics, and thermodynamics. The ability to flow allows fluids to transmit forces and energy, making them essential for systems like pipelines, pumps, and cooling systems.

The other options describe characteristics that do not align with the definition of fluids. For instance, a solid material with a definite shape cannot flow and is thus not a fluid. A gas that cannot be compressed is not an accurate representation, as gases can be compressed, and that characteristic would not apply to all gases. Similarly, a liquid that maintains a constant volume does not fully encapsulate the flexibility to flow and take the shape of its container, which is a fundamental aspect of fluids.