A) Olfactory indicators
B) Litmus indicator
C) Gustatory indicators
D) All of the above
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The correct answer is option A) Olfactory indicators
Olfactory indicators are substances whose odour changes in acidic or basic media. These indicators are used to determine the pH of a solution by simply smelling it.
For example, phenolphthalein is an olfactory indicator that smells sweet in basic solutions, but has no smell in neutral or acidic solutions. Litmus, an extract from certain lichens, is another olfactory indicator which turns red in acidic solutions, blue in basic solutions and purple in neutral solutions.
B) Litmus indicator: Litmus is an olfactory indicator, not a separate category of indicators.
C) Gustatory indicators: Gustatory indicators are substances that change taste in acidic or basic media. They are not commonly used.
D) All of the above: Only Olfactory indicators change their odour in acidic or basic media.