Which phase of matter is characterized by having no definite volume?

The correct answer is gas, and this is due to the unique properties that distinguish gases from other phases of matter. Gases are composed of particles that are widely spaced apart and move freely in all directions. This dispersal allows gases to fill any container they occupy, leading to their lack of a definite volume. Unlike solids, which have tightly packed particles that maintain a fixed shape and volume, or liquids, where particles are closer together but still can flow, gases adapt to the volume of their container. Additionally, plasma, while also not having a definite volume, is a more complex state of matter consisting of ionized gas and is not typically highlighted in defining basic properties of matter related to volume. Thus, gas is the phase of matter that is distinguished by its ability to expand and occupy any volume available to it.