Which of the following configurations would result in a lower impedance than 50 Ω?

In a pure resistance circuit, the impedance is simply equal to the resistance, which is measured in ohms. If the resistance is less than 50 Ω, then the impedance of the circuit will also be less than 50 Ω, since there are no reactive components (like inductors or capacitors) to introduce additional impedance.

In contrast, in a series configuration that includes either inductance (as in a series RL circuit) or capacitance (as in a series RC circuit), the total impedance is increased beyond the value of the resistance due to the reactance contributed by the inductor or capacitor. These reactive components impact the overall impedance calculations, resulting in higher values than that of the pure resistive component alone.

For a parallel RL circuit, the impedance is calculated using both the resistance and the inductive reactance. In this scenario, the presence of the inductor, which contributes additional reactance, will also result in an overall impedance that could potentially be greater than 50 Ω, depending on the values used.

Therefore, the pure resistance circuit is the only configuration where the impedance directly corresponds to the resistance value alone, allowing for the possibility of achieving a value lower than 50 Ω without any contributions from reactive components.