Identifying carboxylic acid from molecular formula and acidity

Steps:

• Molecular formula C4H8O2 fits carboxylic acids (general formula CnH2nO2) or esters.
• Turns damp litmus red, indicating acidity from ionizable H+ (carboxylic acids react with water to form H3O+).
• Exclude esters (neutral, no acidic H).
• For C4H8O2, straight-chain carboxylic acid is butanoic acid; displayed formula shows CH3-CH2-CH2-COOH structure.

Why D is correct:

• D displays CH3-CH2-CH2-COOH, matching C4H8O2 and acidic -COOH group (pKa ~4.8, turns litmus red).

Why the others are wrong:

• A: Ester (e.g., CH3COOCH2CH3), neutral, does not turn litmus red.
• B: Ester (e.g., CH3CH2COOCH3), neutral, lacks acidic proton.
• C: Branched acid (e.g., (CH3)2CHCOOH), acidic but displayed formula differs from straight chain.

Final answer: D