Onion smell remains in one solution and decreases in another. What is the usual nature of the solution in which the smell decreases?

Correct answer: Basic

Onion is an olfactory indicator because its characteristic smell changes in different chemical media. In a basic medium, the onion odour is reduced or becomes difficult to detect, whereas it generally persists in an acidic medium. Thus a decrease in smell indicates a basic solution, making option A correct. This observation is not a test for insolubility, and neutral or acidic descriptions do not match the usual indicator response.