A colloidal system consisting of a gas dispersed in a solid is called a/an:

A colloidal system is classified based on the physical states of the dispersed phase and the dispersion medium. When a gas is dispersed in a solid, the resulting colloidal system is called a solid sol (or solid foam).

Let us review the common types: a gel is a liquid dispersed in a solid, an aerosol is either a liquid or solid dispersed in a gas, and a foam is a gas dispersed in a liquid. When the dispersion medium is a solid and the dispersed phase is a gas, the system is called a solid sol. Common examples include pumice stone and foam rubber, where gas bubbles are trapped within a solid matrix.

Therefore, a gas dispersed in a solid is called a solid sol, which is option (A).