The correct statement about density and states of matter for most substances is:
A. A substance is denser as a solid than as a gas.
In general, solids have a higher density than liquids, and liquids have a higher density than gases. This is because the particles in a solid are closely packed together, whereas in a gas they are much farther apart. While there are exceptions (like water, which is less dense as a solid than as a liquid), the general trend holds for most substances.