Only a lyophilic colloid(i.e. liquid loving colloid) is or can be used as a protective colloid. When added to a lyophilic colloid, it checks the coagulation of a lyophobic colloid(i.e. liquid hating colloid).

Gelatin is lyophilic in nature and hence acts as a protective colloid.

Milk is a kind of emulsion having fats dispersed in an aqueous environment of water.

Suspension contains bigger dispersed particles than colloids which settles down when not disturbed for longer period.

Foam is an emulsion of gas dispersed in liquid medium.

 The Ostwald process can be expressed as a chemical process that is used for making nitric acid whose chemical formula is. Wilhelm Ostwald developed this process in 1902. This process is a mainstay of the modern chemical industry as it provides the main raw material for the most common type of fertilizer production in the world. This process is related to the Haber process, which provides the requisite raw material, ammonia.

This process produces nitric acid. Ammonia is the feedstock for this reaction. It reacts with oxygen and water by using a platinum catalyst (Platinised asbestos) to produce nitric acid. By heating, it is oxidized with oxygen in the presence of a catalyst such as platinum with 10% rhodium, on fuses the platinum metal on silica wool, copper, or nickel, to produce nitric oxide and water vapours. This reaction is considered a strongly exothermic process, making it a useful heat source once initiated.

 Ammonia gets adsorbed more readily on the surface of the charcoal because it is a polar molecule and easily liquifiable. It is due to the critical temperature of ammonia gas which is quite higher than the carbon dioxide. Hence, it easily combines with the materials than carbon dioxide whether it is solid, liquid, or any gases. It has larger Vander Waal's forces of attraction than other elements. Option ‘C’ is the correct answer of the question.

Here, interface of the surfaces means the space between the contact of the two surfaces under consideration. All three, Heterogeneous, homogenous catalysis and corrosion are surface phenomena and so occurs at the interface of the surfaces but crystallisation on the other hand is a bulk phenomena and occurs inside the solution.