Observe the given bar graph comparing the boiling points of the...

In $o$-nitrophenol, the -OH and -NO$_2$ groups are adjacent to each other, which leads to the formation of intramolecular hydrogen bonding.

This intramolecular hydrogen bonding prevents the molecules from forming strong intermolecular hydrogen bonds with each other. As a result, the intermolecular forces are weaker, leading to a lower boiling point.

In contrast, $m$- and $p$-nitrophenol form extensive intermolecular hydrogen bonds, resulting in stronger molecular association and higher boiling points.

Answer: Intramolecular hydrogen bonding