Explanation: A pi-network is a common impedance matching or filter circuit, named because its schematic diagram resembles the Greek letter pi (π). It typically comprises one series reactive component (an inductor or a capacitor) flanked by two shunt reactive components of the opposite type (two capacitors or two inductors, respectively). This configuration is widely used in amateur radio for antenna tuning units (ATUs) to efficiently transfer power from a transceiver to an antenna, and to suppress harmonics. Option A is incorrect because a pi-network specifically refers to a topology of *reactive* components (inductors and capacitors) used for matching and filtering, not just any combination including a resistor, which dissipates power. Option B refers to a phase inversion stage, which is a circuit designed to invert the phase of a signal, typically using active components, unrelated to the passive L-C pi-network topology. Option C refers to a type of computer network architecture (Token Ring), which has no relevance to RF circuit components or amateur radio theory.