Transformer overvoltage burns out filament lamp

Steps:

• Calculate turns ratio: secondary voltage $V_s = 230 \times \frac{20}{300} = 230 \times \frac{1}{15} \approx 15.3$ V.
• Compare to lamp rating: 15.3 V exceeds 6 V maximum by over 2.5 times.
• Initial effect: Cold filament has low resistance, draws high current, lights brightly.
• Subsequent effect: Excessive heat increases resistance but voltage stays high, causing filament to burn out quickly.

Why D is correct:

• Overvoltage exceeds lamp rating, causing initial high current and brightness per Ohm's law ($I = V/R$, low initial R), then filament failure from overheating.

Why the others are wrong:

• A: Lamp lights due to sufficient voltage and current.
• B: Voltage is 2.5 times rating, so brightness exceeds normal.
• C: Overvoltage produces bright light, not dim.

Final answer: D