An "emulsifying agent" is a compound that helps stabilize a hydrophobic colloid in a hydrophilic solvent (or a hydrophilic colloid in a hydrophobic solvent). Which of the following choices is the best emulsifying agent? (a) \(\mathrm{CH}_{3} \mathrm{COOH},\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH},\) (c) \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{11}\) COOH, (d) \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{11} \mathrm{COONa}\).

(Acetic acid) This compound has a hydrophilic carboxyl group (COOH) and a hydrophobic methyl group (CH3). Its hydrophobic part is relatively small compared to other given compounds.

(Butanoic acid) This compound has a hydrophilic carboxyl group (COOH) and a hydrophobic butyl group (CH3CH2CH2). The hydrophobic part is larger than that in Acetic acid, but still smaller than those in the next two compounds.

(Lauric acid) This compound again has a hydrophilic carboxyl group (COOH), but now it has a large hydrophobic part consisting of 12 carbon atoms in a chain. It has a much stronger hydrophobic nature than the previous two compounds.

(Sodium laurate) This molecule has the same hydrophobic part as in Lauric acid with 12 carbon atoms. However, the hydrophilic part in this case is a carboxylate anion (COO-) that is negatively charged and has an ionic nature along with the Na+ ion. It provides a much stronger hydrophilic nature compared to the other compounds with only a COOH group. Now, we need to find the compound that has the best balance between hydrophobic and hydrophilic nature, which would help it act as an effective emulsifying agent. As Sodium laurate (option d) has the significant hydrophobic part and the strongly hydrophilic part due to the ionic nature, this would make it the best emulsifying agent. It will be able to interact with both hydrophobic and hydrophilic parts of the mixture and help in stabilizing it. Thus, the correct answer is: (d) \(\mathrm{CH}_3\left(\mathrm{CH}_{2}\right)_{11} \mathrm{COONa}\).