Which phase of matter is described as a substance that has three dimensions and is firm in substance?

The description identifies a substance that has three dimensions and is firm in substance, which is characteristic of solids. Solids have a defined shape and volume due to the closely packed arrangement of their particles, which are fixed in place and only vibrate slightly. This kind of structural rigidity distinguishes solids from liquids, which take the shape of their container and may flow, and gases, which spread out to fill any available space. A saturated substance, referring typically to a solution, does not specifically imply firmness or three-dimensionality in the same way that solids do. Therefore, the correct answer emphasizes the unique properties of solids as substances that maintain their form and occupy three-dimensional space.