Overvoltage causes filament burnout

Steps:

• Transformer connects 230 V supply to 6 V-rated lamp, likely providing excessive output voltage.
• High voltage causes large current (I = V/R), heating filament intensely.
• Filament glows brightly due to rapid temperature rise.
• Excessive heat melts or vaporizes filament, breaking circuit.

Why D is correct:

• Ohm's law and Joule heating (P = V²/R) show overvoltage produces far more power than rated, leading to bright initial light followed by burnout.

Why the others are wrong:

• A: Lamp lights briefly before failing, so it does light.
• B: Brightness exceeds normal due to overvoltage, not matching rating.
• C: Initial light is bright from high power, not dim.

Final answer: D