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

Explanation

The resonant frequency of an LCR circuit is given by the formula f=1/2π√LC, where L is the inductance and C is the capacitance. To reduce the resonant frequency, the capacitance should be increased, which can be achieved by adding another capacitor in parallel, thus increasing the total capacitance. Option B is correct because it addresses this directly. Option A is incorrect because altering the generator frequency does not change the circuit's inherent resonant frequency. Option C is incorrect because removing the iron core decreases inductance and increases the resonant frequency. Option D is incorrect because removing the dielectric reduces capacitance, which also increases the resonant frequency.