Which of the following compounds has lowest solubility in water?

The solubility of organic compounds in water depends on their ability to form hydrogen bonds with water molecules. Polar groups like $\mathrm{-OH}$ and $\mathrm{-NO_2}$ tend to enhance solubility through hydrogen bonding interactions.

Phenol (A) contains a hydroxyl group capable of forming hydrogen bonds with water, resulting in moderate solubility.

p-Cresol (B) also contains a hydroxyl group but includes a hydrophobic methyl group that increases the non-polar character of the molecule, reducing its hydrogen bonding capacity and solubility compared to phenol.

o-Nitrophenol (C) features both hydroxyl and nitro groups in ortho position. The proximity enables intramolecular hydrogen bonding between these groups, which significantly reduces the availability of the hydroxyl group for intermolecular hydrogen bonding with water. Consequently, o-nitrophenol exhibits the lowest solubility among these compounds.

p-Nitrophenol (D) contains hydroxyl and nitro groups in para position, preventing intramolecular hydrogen bonding. Both groups remain available for strong intermolecular hydrogen bonding with water, resulting in the highest solubility.

The solubility order is p-nitrophenol > phenol > p-cresol > o-nitrophenol.