The correct option is (c) 5 Ω.
Explanation:
Voltage drop across diode (VD) = 0.5 V; Maximum power rating of diode (P) = 100 mW
= 100 x 10-3 W
and source voltage (Vs) = 1.5 V.
The resistance of diode (RD)
And the value of the series resistor = Total resistance of the circuit - Resistance of diode
= 7.5 - 2.5
= 5 Ω