Intramolecular hydrogen bonding occurs when a hydrogen atom within a molecule forms a hydrogen bond with another electronegative atom (e.g., oxygen, nitrogen) present within the same molecule. This bonding is typically observed when the two groups are positioned such that they can interact effectively within the molecule.
Analysis of Each Option:
Option (1):H$_2$O: Water molecules exhibit strong intermolecular hydrogen bonding rather than intramolecular hydrogen bonding.
Option (2): NH$_3$: Ammonia also forms intermolecular hydrogen bonds due to the presence of nitrogen with lone pairs, but it does not exhibit intramolecular hydrogen bonding.
Option (3): C$_2$H$_5$OH (ethanol): Ethanol primarily forms intermolecular hydrogen bonds between different molecules rather than within the same molecule.
Option (4): The compound depicted is ortho-nitrophenol. In ortho-nitrophenol, the hydroxyl (OH) group and the nitro (NO$_2$) group are positioned such that intramolecular hydrogen bonding occurs between the hydrogen atom of the OH group and one of the oxygen atoms of the NO$_2$ group. This interaction stabilizes the molecule and reduces its tendency to form intermolecular hydrogen bonds.
Conclusion:
Ortho-nitrophenol (Option 4) exhibits intramolecular hydrogen bonding.