Which of the following substances acts as an olfactory indicator?

Correct answer: Onion (प्याज)

An olfactory indicator is a substance whose smell changes in an acidic or basic medium. Onion is a standard example: its characteristic smell is altered or becomes difficult to detect in a basic solution, so option A is correct. Turmeric, litmus and phenolphthalein are mainly visual indicators because they show colour changes rather than a smell-based change. The distinguishing clue is the word “olfactory,” which relates to the sense of smell.