Given below are two statements : one is labelled as Assertion and the other is labelled as Reason.
Assertion: At $$10°$$C, the density of a $$5$$ M solution of KCl [atomic masses of K & Cl are $$39$$ & $$35.5$$ g mol$$^{-1}$$ respectively], is 'x' g mL$$^{-1}$$. The solution is cooled to $$-21°$$C. The molality of the solution will remain unchanged.
Reason: The molality of a solution does not change with temperature as mass remains unaffected with temperature.
In the light of the above statements, choose the most appropriate answer from the options given below.

Let us analyze both the Assertion and the Reason.

Assertion: A 5 M solution of KCl at $$10°C$$ has density 'x' g/mL. When cooled to $$-21°C$$, the molality of the solution remains unchanged.

Reason: Molality does not change with temperature as mass remains unaffected with temperature.

Checking the Assertion: Molality is defined as the number of moles of solute per kilogram of solvent:

$$m = \frac{\text{moles of solute}}{\text{mass of solvent (in kg)}}$$

When the temperature changes, the mass of the solute and the mass of the solvent remain the same (mass does not depend on temperature). Therefore, molality does not change with temperature. The assertion is true.

Checking the Reason: The reason states that molality does not change with temperature because mass remains unaffected by temperature. This is exactly why molality is temperature-independent — unlike molarity which depends on volume (which changes with temperature), molality depends only on mass. The reason is true and correctly explains the assertion.

Both the Assertion and Reason are true, and the Reason is the correct explanation of the Assertion.

Hence, the correct answer is Option C.