Q.Which of the following is a constituent of soap?

The main natural source of acetic acid (ethanoic acid) is vinegar.

True — formic acid (methanoic acid, HCOOH) is present in the venom/sting of red ants and bees.

Formic acid, HCOOH, is the simplest carboxylic acid. It is naturally found in the sting and body fluid of ants (its very name comes from the Latin formica, meaning ant) and also in bee stings and stinging nettles. It is also manufactured industrially, but its natur …

Vinegar is a dilute (about 5–8%) aqueous solution of ethanoic (acetic) acid.

Vinegar is obtained by the fermentation of dilute ethanol (or by other biological/industrial routes) to give a dilute aqueous solution containing roughly 5–8% ethanoic acid ($CH_3COOH$), commonly known as aceti …

All three listed substances are genuinely used to preserve food, so the correct choice is "all of these."

• Sodium chloride (common salt): used traditionally to preserve foods like pickles, meat and fish by creating a hypertonic environment that dehydrates and kills microorganisms.
• Sodium metabisulphite: a common chemical preservative used in processed foods, dried fruits, and beverages (an SO$_2$-releasing antimicrobial/antioxidant preservative). …

A soap is, chemically, the sodium salt of a long-chain fatty acid — sodium stearate is exactly this, making it the correct constituent of soap.

Soaps are the sodium or potassium salts of long-chain (typically C₁₆–C₁₈) fatty (carboxylic) acids, made industrially by the saponification (alkaline hydrolysis) of natural fats/oils (triglycerides).

Sodium stearate, $CH_3(CH_2)_{16}COONa$, is the sodium salt of stearic acid (an 18-carbon saturated fatty acid) — this is a textbook example of, and a real constituent of, ordinary soap.

…