Identifying acid P and alcohol Q from isomer constraints Steps:

• Carboxylic acid P has no chain isomers, so P is propanoic acid (formula CH₃CH₂COOH; 3 total carbons prevent branching).
• Alcohol Q has only one positional isomer, so Q is butan-1-ol (straight-chain C4; primary OH position is unique, secondary considered distinct type).
• Ester name is alkyl (from Q) + alkanoate (from P): butyl + propanoate.
• Match to choices: butyl propanoate is option A.

Why A is correct:

• Propanoic acid (CH₃CH₂COOH) has no chain isomers per structural formula, as the ethyl group cannot branch; butan-1-ol has only one positional isomer for primary OH on unbranched chain.

Why the others are wrong:

• B: Butanoic acid has chain isomer (2-methylpropanoic acid, (CH₃)₂CHCOOH).
• C: Propanol has two positional isomers (1-propanol, CH₃CH₂CH₂OH; 2-propanol, CH₃CH(OH)CH₃).
• D: Pentanoic acid has multiple chain isomers (e.g., 2-methylbutanoic acid).

Final answer: A