The units of molality are

We know that $\mathrm{Molality}=\frac{\mathrm{No}. \mathrm{of} \mathrm{moles} \mathrm{of} \mathrm{solute} }{\mathrm{Volume} \mathrm{of} \mathrm{solution}(\mathrm{in} \mathrm{Kg})}$

Therefore, unit of molality are mole ${\mathrm{Kg}}^{-1}$.

The unit of molality is moles per kilogram (mol/kg), which is often written as mol Kg-1$^{-1}$. This means that molality measures the amount of solute (in moles) per unit mass of solvent (in kilograms).

This unit is particularly useful because it does not depend on temperature or volume, unlike molarity, which can change with temperature since it is based on volume.

Thus, the correct answer is option (c), where the unit of molality is mole per kilogram, i.e., mol/kg.