Which of the following conditions is common to both series and parallel resonance?

The condition that is common to both series and parallel resonance is that the resonant frequency occurs when the reactance is equal to zero.

In a series RLC circuit, resonance occurs when the inductive reactance equals the capacitive reactance, causing them to cancel each other out. This results in the impedance of the circuit being at a minimum and the current at a maximum.

In a parallel RLC circuit, resonance occurs when the inductive reactance equals the capacitive reactance, causing them to cancel each other out. This results in the impedance of the circuit being at a maximum and the current at a minimum.

So, in both cases, the condition for resonance is that the inductive reactance equals the capacitive reactance.