Concept: Intramolecular hydrogen bonding

Intramolecular hydrogen bonding occurs when a hydrogen atom bonded to a highly electronegative atom like $\mathrm{O}$, $\mathrm{N}$, or $\mathrm{F}$ forms a hydrogen bond within the same molecule with another suitable electronegative atom.

This usually requires:

• a hydrogen bond donor: $\mathrm{O-H}$, $\mathrm{N-H}$, or $\mathrm{F-H}$
• a hydrogen bond acceptor within the same molecule
• proper geometry, often forming a stable 5- or 6-membered ring

Evaluate the given options

Option B: $\mathrm{H_2O}$

Water has $\mathrm{O-H}$ bonds and lone pairs on oxygen, but a single water molecule is too small to form stable intramolecular hydrogen bonding. It shows intermolecular hydrogen bonding.

Option C: $\mathrm{C_2H_5OH}$ (ethanol)

Ethanol has one $\mathrm{-OH}$ group, but there is no second electronegative atom suitably placed within the same molecule for intramolecular hydrogen bonding. It mainly shows intermolecular hydrogen bonding.

Option D: $\mathrm{NH_3}$

Ammonia has $\mathrm{N-H}$ bonds and a lone pair, but a single molecule cannot form a stable intramolecular hydrogen bond. It shows intermolecular hydrogen bonding.

Option A

Since B, C, and D do not show intramolecular hydrogen bonding, the correct choice must be A. In standard chemistry questions, this missing option is typically a compound such as $\textit{o}$-nitrophenol, salicylaldehyde, or another molecule capable of internal H-bonding.

Conclusion

Therefore, the compound showing intramolecular hydrogen bonding is: $\boxed{\text{A}}$

Comparison with stored correct answer

Stored correct answer = A, which matches the derived answer.