What are the three phases of matter?

The three phases of matter are solid, liquid, and gas. Solids have a definite shape and volume due to closely packed molecules. Liquids have a definite volume but take the shape of their container, allowing the molecules to flow past one another. Gases have neither a definite shape nor volume, as the molecules are far apart and move freely.

This classification is fundamental in understanding the behavior of matter under various conditions. The other options include phases that do not represent the basic states of matter recognized in physics. For instance, plasma is a state that occurs at high energy levels, but it is not one of the three primary states. Vapor refers to a gaseous state of a substance that is typically liquid at room temperature, but it is essentially a gas, not a separate phase. A gel is a colloidal mixture that has properties of both solids and liquids, but it does not constitute a distinct phase of matter. This makes the choice of solid, liquid, and gas the most accurate representation of the three primary phases of matter.