The correct option is Onion.
Explanation
Indicators are substances used to identify the acidic or basic nature of a solution. While most common indicators (like litmus or phenolphthalein) function through a visible color change, olfactory indicators are substances whose characteristic odour changes or vanishes depending on whether the medium is acidic or basic.
Option Analysis
- Onion Onion: Correct. Onion extract serves as a natural olfactory indicator. Its distinct, pungent smell is destroyed in the presence of a base (alkaline solution) but remains detectable in an acidic solution.
- Litmus Litmus: Incorrect. Litmus is a natural visual indicator extracted from lichens. It indicates pH changes through color, turning red in acids and blue in bases.
- Phenolphthalein Phenolphthalein: Incorrect. This is a synthetic visual indicator. It remains colorless in acidic solutions and turns pink in basic solutions.
- Methyl orange Methyl orange: Incorrect. This is a synthetic visual indicator. It turns red in acidic solutions and yellow in basic solutions.
Key Takeaway
Olfactory indicators allow for the identification of acids and bases through changes in smell rather than color. Common examples include onion, vanilla essence, and clove oil, all of which lose their characteristic smell in basic solutions but retain it in acidic solutions.