Correct Option: (a) Both Assertion (A) and Reason (R) are correct and Reason (R) is the correct explanation of Assertion (A).

Step 1 (Governing Principle): [0.5 Mark]

The measurement of resistance of an ionic solution requires an alternating current (AC) source to prevent the phenomenon of electrolysis. If a direct current (DC) source were used, the ions would migrate toward the respective electrodes (anions to anode, cations to cathode), leading to chemical reactions at the electrode surfaces.

Step 2 (Step-by-Step Logical Deduction): [0.25 Mark]

• Effect of DC: Passing DC through an ionic solution causes electrolysis, which changes the composition and the concentration of the electrolyte in the vicinity of the electrodes.
• Change in Resistance: Since the resistance $R$ of an electrolytic solution is dependent on the concentration of ions ($R \propto \frac{1}{\kappa}$, where $\kappa$ is conductivity), any change in concentration leads to an unstable and inaccurate measurement of resistance.
• Role of AC: An AC source reverses the direction of current periodically, preventing the net accumulation of ions at the electrodes, thereby maintaining a constant concentration.

Step 3 (Final Concluded Answer): [0.25 Mark]

Because the DC source alters the concentration of the ionic solution due to electrolysis, it is unsuitable for resistance measurement. Therefore, an AC source is used to ensure the concentration remains constant, validating both the Assertion and the Reason.

💡 EL10 Concept Insight: Think of it like trying to measure the weight of a pile of sand while someone is constantly shoveling it away; if you use DC, the "sand" (ions) moves away, so you use AC to make the ions "dance" back and forth in place so the amount of sand stays the same while you measure it!