Intramolecular lactonization of δ-hydroxy acid

Steps:

• Recognize 5-hydroxypentanoic acid as HO-CH₂-CH₂-CH₂-CH₂-COOH, a δ-hydroxy acid.
• The hydroxyl group attacks the carboxylic acid carbonyl intramolecularly.
• Dehydration occurs, forming a cyclic ester (lactone).
• The resulting structure is a 6-membered δ-lactone ring.

Why C is correct:

• C shows the 6-membered δ-valerolactone, matching the ring size from δ-hydroxy acid cyclization per lactone formation rules.

Why the others are wrong:

• A depicts a 5-membered γ-lactone, incorrect for δ-position OH.
• B shows an open-chain ester, ignoring cyclization.
• D illustrates a 7-membered ring, too large for this molecule.

Final answer: C