To reduce the resonant frequency in an LCR series circuit with a generator:

1. the generator frequency should be reduced
2. another capacitor should be added in parallel to the first
3. the iron core of the inductor should be removed
4. the dielectric in the capacitor should be removed

(b) Hint: The resonant frequency depends on the inductance and the capacitance.
Step 1: We know that resonant frequency in an L -C-R circuit is given by;
ν0=12πLC
Now to reduce ν0, either we can increase L or we increase C.
To increase capacitance, we must connect another capacitor in parallel to the first.