Soap is a type of surfactant, which means it reduces the surface tension of water and allows it to spread over an object or substance more easily. Soap molecules have a polar "head" and a nonpolar "tail". When soap is used in water, the polar heads attract to the water molecules (which are also polar) while the nonpolar tails attract to nonpolar substances like dirt or grease. This allows the soap to effectively lift dirt particles and rinse them away when water is applied.

In acidic conditions, soap molecules undergo a chemical reaction called hydrolysis. This reaction breaks the soap molecule into two components: a long-chain fatty acid and an alcohol. The fatty acid becomes insoluble in water and forms a precipitate, while the alcohol remains soluble.

When the soap molecule is broken down, the nonpolar tail that was responsible for attracting and lifting dirt particles becomes less effective. As a result, the cleansing effect of soap is significantly reduced in acidic conditions.

Therefore, option 4, long-chain fatty acids, is the correct answer. The formation of insoluble long-chain fatty acids makes soap lose its cleansing effect in acidic conditions.