(a) Clove oil and vanilla essence
Olfactory indicators change their smell in acidic or basic media. Clove oil and vanilla essence both show a change in odour with acids and bases, making them olfactory indicators.
The textbook (Activity 2.2, Chapter 2) states that onion, vanilla essence, and clove oil are tested as olfactory indicators. However, onion is also confirmed as one, but turmeric is a colour-based indicator. Litmus and methyl orange are synthetic colour indicators — not olfactory. The pair that consists of only olfactory indicators is clove oil and vanilla essence (option a). Students often confuse turmeric and onion — remember turmeric is a colour indicator, not olfactory.