An emulsifier is an agent which:
A. accelerates the dispersion
B. homogenizes an emulsion
C. stabilizes an emulsion
D. aids the flocculation of an emulsion

Hint: Emulsification relates to the phenomenon in which two generally immiscible liquids form a mixture. Thus, since these liquids will have a natural tendency to separate out into their pure states, certain other compounds called emulsifying agents need to be added to prevent this from happening.

Complete answer:
First let us understand what colloids are:
Colloids are formed by the mixing of dispersion medium and the dispersed phase which are neither true solutions nor suspensions. The dispersed medium in a colloidal system refers to the medium in which colloidal particles are dispersed while the dispersed phase is the phase dispersed through the medium. Together, dispersed phase and dispersion medium form colloidal systems or colloids.
Coming to the term ‘emulsion’, we see that:
An emulsion is a kind of colloidal solution in which both dispersed phase and dispersion medium are liquids. Since the two liquid pairs are immiscible with each other, we can understand that emulsions are naturally unstable in nature.
Therefore, for the purpose of stabilizing the emulsion, an emulsifying agent or an emulsifier is added to the emulsion. For example, milk is an emulsion with liquid fat as dispersed phase and liquid water as dispersion medium and casein is the emulsifier.
Hence, option (C) is the correct option.

Additional Information: There are numerous applications of emulsions like:
-Digestion of fat in small intestines occurs easily due to emulsion
-In metallurgy, the concentration of ore by froth floatation is based upon emulsion.
-Soaps and detergents are used as cleansing agents by forming emulsions.
-Disinfectants like Dettol and phenyl when mixed with water form emulsions.
-Even for the concentration of ores, when the finely powdered ore is mixed with oil it forms emulsion.

Note: The major role of emulsifying agents is to form a layer over the globules which are dispersed in the solvent, and to reduce the tension at the interface between the two liquids. A number of theories have been proposed to explain the mechanism of emulsification, like the surface tension theory and the repulsion theory.