A liquid was mixed with ethanol and a drop of concentrated $$H_2SO_4$$ was added. A compound with a fruity smell was formed. The liquid was:

When ethanol is heated with a liquid in the presence of a few drops of concentrated $$H_2SO_4$$, we should recall the Fischer esterification reaction:

Carboxylic acid $$+$$ alcohol $$\xrightarrow{conc\,H_2SO_4,\,\Delta}$$ ester $$+$$ water

Ester molecules are well-known for having pleasant, fruity odours (banana, pineapple, apple, etc.). Therefore, if a “compound with a fruity smell” appears, the unknown liquid must be a carboxylic acid.

Now examine each option:

Option A $$CH_3OH$$ (methanol) is an alcohol, not a carboxylic acid. With another alcohol (ethanol) it would undergo dehydration to give ethers, not esters. No characteristic fruity smell would result.

Option B $$HCHO$$ (formaldehyde) is an aldehyde. In acidic ethanolic medium it tends to form acetals/hemiacetals, again not associated with strong fruity odours.

Option C $$CH_3COCH_3$$ (acetone) is a ketone. Ketones do not esterify with alcohols under these conditions.

Option D $$CH_3COOH$$ (acetic acid) is a carboxylic acid. With ethanol and concentrated $$H_2SO_4$$ it forms ethyl ethanoate (ethyl acetate):

$$CH_3COOH + C_2H_5OH \xrightarrow{conc\,H_2SO_4} CH_3COOC_2H_5 + H_2O$$

Ethyl acetate possesses a characteristic sweet, fruity smell, exactly matching the observation in the question.

Hence the liquid must be acetic acid.

Option D which is: $$CH_3COOH$$