The emulsifying agent in milk is:
A.Lactic acid

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Hint: To answer this question we should know about emulsifying agents. We add a small amount of substance (called an emulsifying agent) to the emulsion during its preparation. Now try to find what could be an emulsifying agent in milk.

Complete step by step answer:
-First, we will talk about the emulsion. The emulsion is a mixture of two liquid substances that normally don't mix with each other due to opposite physical characteristics.
- To prevent breaking of emulsions, a small amount of third substance is added to the emulsion during its preparation, which is called an emulsifying agent or emulsifier
 -Milk (it is an emulsion of liquid fats in water) is stabilized by casein (an emulsifying agent) which is a protein containing phosphate groups. The primary emulsifying agent in dairy emulsions, is the proteins (i.e. caseins, in either monomeric or micellar form). These proteins adsorb at the oil/water or air/water interface, protecting the interfacial membranes from coalescence.

Therefore, we can conclude that the correct answer to this question is option D.

Additional information:
Generally soaps, detergents, long-chain sulphonic acids, lyophilic colloids, alkyl sulfates, etc. are used as emulsifying agents. These compounds form a protective layer surrounding the dispersed phase and inhibit the particles to come closer and thus reduce the tendency of particles to coagulate.

We should also know that Emulsifiers that are more soluble in water (and conversely, less soluble in oil) will generally form oil-in-water emulsions, while emulsifiers that are more soluble in oil will form water-in-oil emulsions.