Which of the following is an example of an olfactory indicator?

Olfactory indicators are substances that change their smell in the presence of acids or bases. Onion is an olfactory indicator because its characteristic smell diminishes in the presence of a base but remains unchanged in an acidic medium. This property helps distinguish between acidic and basic solutions through the sense of smell. Other examples of olfactory indicators include vanilla extract and clove oil, which behave similarly. In contrast, turmeric, China rose, and methyl orange are visual pH indicators that change color rather than smell in different pH environments.