Which one of the following can be used as an acid-base indicator by a visually impaired student?

Vanilla essence serves as an olfactory indicator. An olfactory indicator is a substance whose smell differs in when added to acids and bases respectively. Vanilla essence has a characteristic pleasant smell. When added to a basic solution such as sodium hydroxide solution, the characteristic smell of vanilla essence vanishes. When added to an acidic solution such as hydrochloric acid, however, the characteristic pleasant smell of vanilla essence remains. This is how it helps such students to identify acids and bases.