Colligative properties — such as relative lowering of vapour pressure, elevation of boiling point, depression of freezing point, and osmotic pressure — depend only on the number of solute particles present in a solution, not on their chemical identity. Because these properties are frequently measured across a range of temperatures (for instance, a solution is heated to determine its boiling point, or cooled to determine its freezing point), the concentration unit used in the calculations must itself remain reliable and unchanged as temperature varies. This is where the choice between molarity and molality becomes important.

Molarity is defined in terms of the total volume of the solution. Volume is a temperature-dependent quantity — liquids expand on heating and contract on cooling (thermal expansion), so the same solution will have a slightly different volume, and therefore a different molarity, at different temperatures even though the actual amount of solute and solvent has not changed. Molality, on the other hand, is defined in terms of the mass of the solvent, and mass does not change with temperature (mass is conserved regardless of thermal expansion or contraction). This makes molality a temperature-independent measure of concentration.

Since colligative property experiments inherently involve a change in temperature (that is the whole point of measuring boiling point elevation or freezing point depression), using molarity would introduce an artificial, temperature-driven error into the concentration value itself, distorting the calculation. Molality avoids this problem entirely, which is why it is the preferred unit in these calculations, and why the correct choice is that molality remains unaffected by temperature changes unlike molarity.

The other options are inconsistent with these facts: it is not true that molality depends on volume changes with temperature — molality is deliberately defined in terms of mass precisely to avoid such dependence; the claim that molarity stays constant while molality varies with temperature is the reverse of the actual physical behaviour; and it is not true that both remain unaffected by temperature, since molarity is explicitly volume-based and therefore does vary as the solution's volume changes with temperature.