The three states of matter are solid, liquid, and:

The three primary states of matter are solid, liquid, and gas. In a solid state, particles are tightly packed, giving them a definite shape and volume. In a liquid state, particles are close together but can move freely, allowing liquids to take the shape of their container while maintaining a constant volume. In the gas state, the particles are spaced far apart and move freely, allowing gas to expand to fill any container, resulting in neither a definite shape nor volume.

The selection of gas as the third state of matter is foundational in understanding basic physical science. Each state responds differently to changes in temperature and pressure, which is critical when studying the behavior of substances in various environments, such as in fire dynamics or firefighting applications. The other choices, though relevant in different contexts, do not represent fundamental states of matter: ice is a solid form of water, air consists of a mixture of gases, and antimatter refers to particles that are counterparts to conventional matter, not a classic state of matter.