Terminal voltage rises with decreasing current in cells with internal resistance

Steps:

• Terminal voltage $V = E - Ir$, where $E$ is emf, $I$ is current, and $r$ is internal resistance.
• Current $I = E / (R_\text{total} + r)$, with $R_\text{total}$ as external resistance including fixed $R$ and potentiometer.
• Moving Z towards X increases the effective potentiometer resistance, raising $R_\text{total}$.
• Higher $R_\text{total}$ lowers $I$, reducing $Ir$ drop, so $V$ increases.

Why C is correct:

• By Ohm's law, $V = E \cdot \frac{R_\text{total}}{R_\text{total} + r}$; increasing $R_\text{total}$ makes $V$ approach $E$.

Why the others are wrong:

• A: Voltmeter reading actually increases, not decreases, when current falls.
• B: Current increase would decrease reading, but motion causes current decrease.
• D: Current decrease causes reading increase, not the reverse.

Final answer: C