Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : An aqueous solution of KOH when for volumetric analysis, its concentration should be checked before the use.
Reason (R) : On aging, KOH solution absorbs atmospheric $$CO_2$$.
In the light of the above statements, choose the correct answer from the options given below.

Assertion (A) states that when an aqueous solution of KOH is used for volumetric analysis, its concentration should be checked before use. This is correct because KOH is a strong base that readily reacts with atmospheric $$CO_{2}$$, causing its effective concentration to change over time.

Reason (R) states that on aging, KOH solution absorbs atmospheric $$CO_{2}$$. This is also correct. The reaction that takes place is $$2\,KOH + CO_{2} \rightarrow K_{2}CO_{3} + H_{2}O$$. As $$KOH$$ gets consumed in this reaction, the actual concentration of $$KOH$$ in the solution decreases with time.

Since the absorption of $$CO_{2}$$ (Reason) directly explains why the concentration of $$KOH$$ changes and must be re-checked (Assertion), the Reason is indeed the correct explanation of the Assertion.

Hence, the correct answer is Option C.