There are some substances whose odor changes in acidic or basic media. These are called - (1) Olfactory indicators (2) Litmus indicator (3) Gustatory indicators (4) All of the above
(1) Olfactory indicators Explanation: An Olfactory indicator is a substance whose smell varies depending on whether it is mixed with an acidic or basic solution. Onion , clove oil and vanilla extract are examples.