All the three classes of aliphatic amines form H-bonds with water. As the size of alkyl group increases, the solubility decreases due to a corresponding increase in the hydrophobic part (hydrocarbon part) of the molecule. On the other hand, aromatic amines are insoluble in water due to presence of larger hydrocarbon part (C₆H₅-group) which tends to retard the formation of H-bonds.

Now, among the given compounds, C₆H₅NH₂ is insoluble in water due to presence of C₆H₅-group (hydrocarbon part). In (C₂H₅)₂NH, two alkyl groups are present and in C₂H₅NH₂ only one alkyl group is present, hence C₂H₅NH₂ is more soluble in water than (C₂H₅)₂NH.

Combining all these facts, solubility of the given three compounds increases in the order:

C₆H₅NH₂ < (C₂H₅)₂NH < C₂H₅NH₂