Which of the following is not an olfactory indicator?

Correct answer: C) Sweet potato

The substance which is NOT an olfactory indicator is sweet potato.

Explanation:
Olfactory indicators are substances whose smell changes in acidic or basic solutions.

• Clove oil – olfactory indicator

• Vanilla – olfactory indicator

• Sweet potato – not an olfactory indicator

Hence, the correct answer is C) Sweet potato.