What is a defining feature of gases?

A defining feature of gases is that they can be compressed easily. This characteristic is due to the large amount of space between gas particles compared to the particles in liquids and solids. In a gas, the particles are in constant random motion and are far apart, allowing them to be squeezed or packed closer together when pressure is applied. This property is a significant reason why gases can fill any container they are placed in, adapting to the shape and size of that container.

In contrast, the other options describe properties that are typical of solids or liquids rather than gases. For example, having a definite shape and volume is characteristic of solids, while maintaining a constant volume regardless of temperature applies to solids and some liquids, not gases. Lastly, occupying a fixed space contradicts the nature of gases, which readily expand to fill the entirety of their container. Thus, the ability to be compressed easily is a fundamental trait that distinguishes gases from other states of matter.