Which substance is not used for preparing lyophilic sols?
A.Starch
B.Gum
C.Gelatin
D.Metal sulphide

Hint: Mostly natural organic substance forms lyophilic sol like gum, albumin and cellulose solution. But mostly inorganic substances like gold, arsenic form lyophobic sol.

Complete step by step answer:
Colloidal solution in which the dispersed phase has considerable affinity for the dispersion phase is called lyophilic sol. The examples include Dispersion of gelatine, starch, Gum and protein in water. Colloidal solutions in which the dispersed phase has no affinity or attraction for the medium or for the solvent are called lyophobic colloid solutions.
Lyophilic sol can be prepared by shaking the substance with solvent. Lyophilic sols are more stable, they are reversible, their viscosity is very high but the surface tension is very low. The particles have little charge or there may be no charge at all, particles cannot be seen under a microscope. They can be precipitated by high concentration of electrolytes; they do not show tyndall effect. For example gelatin, starch gum, Albumin and cellulose solution.
Lyophobic sol, they cannot be prepared easily by shaking and they require special methods for their preparation, they are less stable than lyophobic sol and are Irreversible. Viscosity is nearly equal to solvent and also the surface tension is as same as solvent. The particle carry charge may be positive or may be negative. Their particles can be seen under a microscope. They can even precipitate by Low concentration of electrolyte, they show tyndall effect. For example: gold and arsenic.
The Dispersion of apparently precipitated material into colloidal solution by the action of an electrolyte in solution is termed as peptization, the electrolyte used is called peptizing agent.

Thus, the correct option is D.

Note:
 Hardy Schulze rule states that the precipitating effect of an ion on a dispersed phase of opposite charge increases with the valency of the ion. The minimum concentration of an electrolyte in millimoles required to cause precipitation of 1 litre sol in 2 hours is called flocculation value. The number of milligram of hydrophilic colloids and that will just prevent the precipitation of 10 ml of standard gold sol on the addition of 1 ml of 10 percent sodium chloride solution is called as gold number.