Oxidation of primary alcohols yields carboxylic acids Steps:

• Identify butanoic acid formula: CH₃CH₂CH₂COOH (4-carbon chain).
• Examine A: Acid-base reaction of CH₃CH₂COONa yields CH₃CH₂COOH (propanoic acid, 3 carbons).
• Examine B: Hydrolysis of CH₃CH₂CN (propanenitrile) yields CH₃CH₂COOH (propanoic acid).
• Examine C: Acidic hydrolysis of CH₃CH₂COOCH₃ (methyl propanoate) yields CH₃CH₂COOH (propanoic acid).
• Examine D: Oxidation of CH₃CH₂CH₂OH (butan-1-ol) with strong oxidant like KMnO₄ yields CH₃CH₂CH₂COOH (butanoic acid).

Why D is correct:

• Primary alcohols oxidize to carboxylic acids with the same carbon chain length, per standard organic oxidation rules.

Why the others are wrong:

• A produces propanoic acid due to 3-carbon starting material.
• B produces propanoic acid from nitrile hydrolysis adding O but retaining 3 carbons.
• C produces propanoic acid from ester hydrolysis retaining 3-carbon acyl group.

Final answer: D