The correct choice is (d) 1/3
Explanation: The response curve for the lead-lag circuit indicates that the output voltage peaks at a frequency called the resonant frequency, fr. At this point, the attenuation (Vout/Vin) of the circuit is 1/3.
Vout/Vin = 1/3
The lead-lag circuit in the Wien-bridge oscillator has a resonant frequency fr, at which the phase shift through the circuit is and the attenuation is 1/3.