Internal resistance affects current, not e.m.f.

Steps:

• Electromotive force (e.m.f.) is the cell's open-circuit potential difference, unchanged by internal resistance.
• Circuit current I = E / (R + r), where E is e.m.f., R is fixed external resistance, and r is internal resistance.
• As r increases, total resistance (R + r) rises, so I decreases.
• Power dissipated in fixed resistor P = I²R; with I decreasing, P decreases.

Why C is correct:

• e.m.f. E is intrinsic to the cell and unaffected by r (definition of e.m.f.); power P decreases per P = I²R formula as I falls.

Why the others are wrong:

• A: e.m.f. does not decrease; it remains constant.
• B: e.m.f. does not decrease; it remains constant.
• D: power dissipated decreases, not unchanged.

Final answer: C