Which of the following best defines gas?

The definition of gas as a material with a tendency to expand indefinitely accurately captures the essential characteristics of gases. Unlike solids and liquids, gases do not have a fixed volume or shape; they fill the container they are in, adapting to its dimensions. This expansion ability is due to the high kinetic energy of gas molecules, which are in constant motion and colliding with each other and the walls of their container. This behavior contrasts starkly with the properties of solids, which maintain fixed shapes and volumes, and liquids, which have fixed volumes but take the shape of their containers. Therefore, option B correctly encapsulates the fundamental nature of gases in contrast to other states of matter.