An example of solid sol is:

First, we recall the classification of colloidal systems. In a colloid, we always speak of two parts — the dispersed phase and the dispersion medium. The different possible physical states of these two parts give rise to different special names:

$$\begin{array}{c|c|c} \text{Dispersion medium} & \text{Dispersed phase} & \text{Name of colloid} \\ \hline \text{Gas} & \text{Liquid} & \text{Aerosol\,(fog)} \\ \text{Liquid} & \text{Solid} & \text{Sol\,(paint, inks)} \\ \text{Liquid} & \text{Liquid} & \text{Emulsion\,(milk)} \\ \text{Solid} & \text{Liquid} & \text{Gel\,(butter, cheese)} \\ \text{Solid} & \text{Solid} & \textbf{Solid sol\,(gem stones, coloured glass)} \\ \end{array}$$

From the table we see that a solid sol is specifically a colloidal system in which both the dispersion medium and the dispersed phase are in the solid state. Typical textbook examples include coloured or smoky glass, ruby, sapphire, and other gem stones where tiny particles of one solid are uniformly distributed throughout another solid matrix.

Now we match each option with the correct colloidal category:

• Paint: here solid pigment particles are dispersed in a liquid medium such as water or oil, so paint is an ordinary sol, not a solid sol.

• Gem stones: minute particles of colouring metal oxides (solid) are dispersed in a solid silica or alumina lattice (solid). This fits the definition $$\text{solid in solid} \rightarrow \text{solid sol}.$$

• Butter: this is an example of a gel or an emulsion where tiny liquid droplets of water and fat are trapped in a mostly solid fat matrix. It is classified as a gel, not a solid sol.

• Hair cream: this is a thick liquid containing small liquid fat/oil droplets dispersed in water; therefore it is an emulsion, again not a solid sol.

By elimination and by direct application of the definition, only gem stones satisfy the criterion for a solid sol.

Hence, the correct answer is Option B.