A parallel plate capacitor separated by an air gap is made of 0.8 m2 tin plates and 20 mm

For a parallel plate capacitor, C = ε₀A/d. Here d = 20 mm = 0.02 m.

C = (8.85 × 10⁻¹² × 0.8)/0.02 = 3.54 × 10⁻¹⁰ F.

Charge Q = CV = 3.54 × 10⁻¹⁰ × 120 = 4.25 × 10⁻⁸ C = 42.5 nC. The 3.54 nC option comes from forgetting to multiply by 120 V correctly or using the wrong power of ten.