What best describes gas in terms of its state of matter?

Gas is characterized by having neither a fixed shape nor a fixed volume. Instead, it expands to fill the entirety of its container, regardless of the container's size or shape. This unique property stems from the fact that the molecules in a gas are in constant, rapid motion and are far apart from each other, allowing them to move freely. Unlike solids, which maintain a defined shape due to tightly packed molecules, or liquids, which have a fixed volume but adopt the shape of their container, gases lack both of these properties. Therefore, the correct description emphasizes their adaptability to any container's shape while not being confined to a set volume.