Soaps are:

Soap is chemically defined as the alkali metal salt of a higher fatty acid. Sodium or potassium (the alkali metals) combine with the anion of a long-chain (12 or more carbon atoms) carboxylic acid — the ‘higher’ fatty acid — to form a soap molecule. Soaps are manufactured by saponification: a fat or oil (a triglyceride ester of higher fatty acids) is boiled with an alkali such as NaOH or KOH, which splits the ester and releases the fatty acid as its alkali-metal salt, together with glycerol.

Matching this definition against the given description, the correct answer must combine two features together: the cation must come from an alkali metal (sodium or potassium), and the fatty acid must be a higher (long-chain) one, not a lower (short-chain) one. ‘Alkali metal salts of higher fatty acids’ is exactly this combination, so it is the accurate chemical description of soap.

Checking against each option:

• Alkali metal salts of lower fatty acids describes too short a hydrocarbon chain — a lower fatty acid does not have the long, oil-loving tail that gives soap its cleansing (surfactant) action.

• Dodecyl benzene sulphonate is a sulphonate salt of an alkylbenzene compound. It is the class of substance used to make synthetic detergents, produced without any fat, oil, or saponification step — chemically a different family from soap altogether.

• Alkaline metal salts of higher fatty acids uses the general adjective ‘alkaline’ (meaning basic in nature) instead of the precise term ‘alkali metal’ (the specific Group 1 elements). A substance can be alkaline without its cation being an alkali metal at all, so this phrasing does not correctly capture soap’s composition.

Alkali metal salts of higher fatty acids therefore correctly defines soaps.