Stabilization by intramolecular hydrogen bonding occurs when a hydrogen atom is bonded to an electronegative atom (such as oxygen or nitrogen) and forms a hydrogen bond with another electronegative atom within the same molecule. This type of bonding can lead to unique structural and chemical properties of the molecule.

Let's analyze each option to determine if it exhibits intramolecular hydrogen bonding:

a) o-nitrophenol:
In o-nitrophenol, the hydrogen atom bonded to the hydroxyl group (-OH) can form a hydrogen bond with the oxygen atom of the nitro group (-NO2). This intramolecular hydrogen bonding stabilizes the molecule by forming a ring-like structure, enhancing its stability.

b) p-nitrophenol:
In p-nitrophenol, the hydrogen atom bonded to the hydroxyl group (-OH) does not have an electronegative atom nearby to form an intramolecular hydrogen bond. Therefore, this molecule does not exhibit intramolecular hydrogen bonding.

c) Acetyl acetone in enolic form:
In the enolic form of acetyl acetone, the hydrogen atom bonded to the hydroxyl group (-OH) can form an intramolecular hydrogen bond with the oxygen atom of the carbonyl group (-C=O). This hydrogen bond stabilizes the molecule and influences its tautomeric equilibrium.

d) Benzoic acid:
In benzoic acid, the hydrogen atom bonded to the carboxyl group (-COOH) can form an intramolecular hydrogen bond with one of the oxygen atoms of the carbonyl group (-C=O). This hydrogen bond stabilizes the molecule and influences its structure and reactivity.

Overall, the molecules that are stabilized by intramolecular hydrogen bonding are o-nitrophenol and acetyl acetone in enolic form (option A and option C). These hydrogen bonds play a crucial role in the stability, structure, and properties of these molecules.

A