Olfactory indicator helps to identify whether the given solution is acidic or basic by changing:

A. Color

B. Smell

C. Texture

D. All of the above

Olfactory indicators are those indicators that help to identify whether the given solution is acidic or basic by changing their smell instead of colour as the other indicators do. Olfactory indicators are based on the sensation of smell to identify the nature of the substance.

An olfactory indicator works on the principle that when an acid or base is added to it, then different odour can be detected in bases whereas the odour remains the same in acids. Certain substances that change its odour in Acidic or basic medium. An acid-base indicator is either a weak acid or weak base that exhibits a color change as the concentration of hydrogen ($H^+$) or hydroxide ($OH^-$) ions changes in an aqueous solution. They are also used to gauge pH values and for interesting color-change science demonstrations.

Now, let us try to understand with few examples:

Onion: It poses its characteristic smell in basic solution and retains its smell in Acidic solution.

Vanilla: It loses its smell in basic solution and retains its smell in Acidic solution.

Clove: Even clove loses its smell in basic solution and retains its smell in Acidic solution.