Voltage drop across internal resistance

Steps:

• Terminal p.d. V equals e.m.f. E minus voltage drop across internal resistance I r.
• Adjusting variable resistor decreases its resistance, increasing total current I.
• Higher I causes larger voltage drop I r across internal resistance r.
• Thus, V = E - I r decreases as I r increases.

Why C is correct:

• By Ohm's law, p.d. across internal resistance is I r, which increases with I, reducing terminal p.d. V.

Why the others are wrong:

• A: E.m.f. E is constant for the battery.
• B: Internal resistance r is fixed.
• D: Variable resistor decreases to increase I.

Final answer: C