LDR triggers lamp via potential divider in darkness

Steps:

• LDR resistance rises in no light, increasing voltage drop across it in a divider.
• Potential divider with LDR and fixed resistor feeds transistor base for switching.
• High LDR resistance in dark must output voltage to turn on NPN transistor and lamp.
• LDR resistance less than lamp ensures divider doesn't overload lamp circuit.

Why B is correct:

• B uses LDR on top of divider; dark raises junction voltage per Ohm's law (V = IR), activating transistor to switch lamp on.

Why the others are wrong:

• A: LDR bottom of divider; dark lowers voltage, keeping lamp off.
• C: No transistor switch; lamp always on or direct short risk.
• D: Fixed resistor top; dark lowers voltage, inverting trigger logic.

Final answer: B