To reduce the resonant frequency in an LCR series circuit with a generator.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → another capacitor should be added in parallel to the first
Adding a capacitor in parallel increase the effective capacitance $(C_{eff}>C)$.
Since $f_r∝\frac{1}{\sqrt{C}}$ increasing C will decrease $f_r$.