Select the compound from the following that will show intramolecular hydrogen bonding:

🧠 Intramolecular hydrogen bonding forms within the same molecule — typically when an O–H or N–H group and a lone-pair-bearing atom are on the same molecule, close enough to form a ring.

🗺️ Analyse each option:

• $\mathrm{H_2O}$: only intermolecular H-bonding (no intramolecular geometry possible)
• $\mathrm{NH_3}$: only intermolecular H-bonding
• $\mathrm{C_2H_5OH}$: only intermolecular H-bonding (no nearby acceptor group)
• o-nitrophenol: The phenolic –OH and the ortho $\mathrm{–NO_2}$ group are on adjacent carbons. The O–H hydrogen bonds to one oxygen of $\mathrm{–NO_2}$, forming a 6-membered intramolecular ring → intramolecular H-bonding ✓

⚡ This is why o-nitrophenol has a lower melting and boiling point than p-nitrophenol: the intramolecular H-bond "uses up" the donor, reducing intermolecular associations.

$\boxed{\text{Answer: o-nitrophenol}}$