While preparing soap, a small quantity of common salt is generally added to the reaction mixture of vegetable oil and sodium hydroxide. Which one of the following may be the purpose of adding common salt?

Hint: The process involved in the reaction is called saponification. It is alkaline hydrolysis of esters where the product is formed as a precipitate. Common salt is added to the solution containing the sodium salt of the ester produced to convert it into soap.

Complete step-by-step answer:
Soaps are sodium or potassium salts of long-chain fatty acids. When fat/oil (triglycerides) react with aqueous NaOH or KOH, they are converted into soap and glycerol. This is called alkaline hydrolysis of esters.
The reaction occurs as follows:
$R - COOR\xrightarrow{{NaOH}}R - COONa + R - OH$
    Ester Soap Alcohol
In the reaction product formed are usually sodium salts of fatty acids so when salt is added while preparation it will introduce common ion effect into the solution due to which concentration of common ion (sodium) will increase on the reactant side thus make the reaction to go towards product side thus will increase the precipitation of soap.
The alkyl group consists of very long chains (nearly 20 to 30 carbon atoms) which is hydrophobic and the head containing sodium ions is hydrophilic.
Hence, we can say soap is added in order to favor the precipitation of soap.

Note: Saponification: conversion of fat, oils, and lipids into alcohol and soap using aqueous alkali. Saponification is an exothermic process.
Common ion effect: effect on equilibrium that occurs when a common ion is added to a solution. Common ions generally decrease solubility.