Which of the following acts as an olfactory indicator in acid-base reactions?

Onion extract is an olfactory indicator. These substances identify acids and bases through a change in smell/odour rather than colour. In acids, onion extract retains its characteristic pungent smell. In bases, the characteristic smell of onion disappears or becomes undetectable. Other olfactory indicators include vanilla essence and clove oil. Like onion, they lose their distinct smell in basic solutions but retain it in acidic ones.