Lamp brightness maximizes with highest voltage across parallel-connected identical lamps Steps:

• Identical cells provide emf E each; lamps have resistance R each.
• Series lamps divide total voltage equally; parallel lamps each receive full circuit voltage.
• Cells in series add emfs for higher total voltage; parallel cells maintain emf E.
• Power per lamp P = V²/R, where V is voltage across it; highest V yields brightest lamps. Why D is correct:
• Two cells in series give 2E total; parallel lamps each across 2E, so P = (2E)²/R = 4E²/R by power formula. Why the others are wrong:
• A: One cell (E total), series lamps; each V = E/2, P = E²/(4R), lowest.
• B: One cell (E total), parallel lamps; each V = E, P = E²/R, half of D.
• C: Two cells in series (2E total), series lamps; each V = E, P = E²/R, half of D.

Final answer: D