Which of the following represents a resonant or tuned circuit?

A resonant or tuned circuit is achieved by the combination of an inductor and a capacitor, which can be arranged either in series or in parallel. This setup allows the circuit to oscillate at a specific frequency, known as the resonant frequency. When the circuit is at this frequency, the inductive and capacitive reactances cancel each other out, leading to a maximum current flow in a series arrangement, or maximum voltage in a parallel arrangement.

The presence of both inductance and capacitance enables the tuning of the circuit to a desired frequency, making it useful for applications such as radio transmitters and receivers, where selecting or filtering specific frequencies is essential. This property also enables the circuit to store energy oscillating between the magnetic field of the inductor and the electric field of the capacitor, further reinforcing the tuned circuit’s ability to resonate at the chosen frequency.

In contrast, other options like linear voltage regulators, resistor circuits to reduce standing wave ratios, or high-fidelity audio circuits do not inherently rely on the resonance phenomenon that characterizes a tuned circuit. These alternatives focus on different functions and designs, and do not utilize the reactance interactions central to resonant circuits.