On what basis do olfactory indicators help identify acids and bases?

Correct answer: By change in smell

Olfactory indicators are substances whose characteristic smell changes, becomes weak, or is masked in acidic or basic media. This smell response provides evidence about the nature of the tested solution without relying on a colour change. Onion, vanilla essence, and clove are commonly cited school-level examples. Thus option B is correct. Permanent black colour, metal formation, and zero temperature are not the identifying basis of these indicators.