Answer: The correct answers are option(b) and(d).
Explanation:
When the lamp was dimmer the resistance offered by the lamp was more which just resist the flow current from the circuit.
(Ohm's Law)
When the Janet decrease the resistance to (R') by sliding the dimmer switch (voltage remained constant). On decreasing the resistance the flow of current in circuit will increase by which light of the lamp will become brighter.
Hence, from the enlisted options the correct options option(b) and(d).