What characteristic is true of gases?

Gases are unique in that they do not have a fixed shape or a fixed volume. Instead, they will expand to fill the entire volume and shape of their container. This property is a result of the high kinetic energy of gas molecules, which allows them to move freely and spread out to occupy the available space. This behavior is in contrast to solids and liquids, which have defined shapes and volumes due to their molecular structures and intermolecular forces.

The behavior of gases is fundamental in various applications, including fire dynamics and explosion investigations, as understanding how gases behave in different environments is vital for assessing fire spread, developing fire suppression strategies, and understanding the dynamics of explosions.

In the context of the other choices, gases do not have a fixed shape or volume (which rules out the first option), they can exist in a wide range of temperatures (making the third option inaccurate), and not all gases are flammable (as many gases are inert or non-combustible). Therefore, the correct characteristic that describes gases in this context is that they will expand to fill the shape and volume of their container.