Terminal voltage drop from internal resistance

Steps:

• Terminal p.d. V = emf - I r, where I r is the voltage drop across internal resistance.
• Adjusting the variable resistor increases I by decreasing its value.
• Larger I causes larger voltage drop I r across internal resistance r.
• Thus, V = emf - larger I r decreases.

Why C is correct:

• By Kirchhoff's voltage law, V = emf - (p.d. across internal resistance); increasing I raises this p.d. (I r), lowering V.

Why the others are wrong:

• A: Battery emf remains constant regardless of current.
• B: Internal resistance r is fixed and does not change.
• D: Increasing I requires decreasing the variable resistor's resistance.

Final answer: C