Internal resistance reduces terminal voltage but not emf

Steps:

• E represents the electromotive force (emf), the total energy per unit charge supplied by the cell.
• Replacing the cell keeps the emf the same, so E remains unchanged.
• V is the terminal voltage across the external circuit, given by V = E - Ir, where I is current and r is internal resistance.
• Significant internal resistance increases the Ir drop, reducing V while current I stays the same for the complete circuit.

Why C is correct:

• Emf E is defined as the open-circuit voltage and unaffected by internal resistance; terminal voltage V decreases per V = E - Ir as r increases.

Why the others are wrong:

• A: E does not decrease, as emf is unchanged.
• B: E does not decrease, as emf is unchanged.
• D: V does not increase, as added resistance causes greater internal drop.

Final answer: C