Each pair forms ideal solution except

Take note of the differences in the nature of intermolecular forces among the given options.
We can see that the intermolecular forces in options A and B are the same, that is dipole-dipole interaction. There is the presence of London forces in both molecules of Option C.

However, in Option D, molecule ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{I}$ has a dipole-dipole interaction, while molecule ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}$ has hydrogen bonding. As a result, the nature of intermolecular forces differs here.

Therefore, the required pair forming non-ideal solution is D)${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{I}$ and ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}$.