The correct answer is D: Precision printed circuit conductors above a ground plane that provide constant impedance interconnects at microwave frequencies. Microstrip is precision printed circuit conductors above a ground plane that provide constant impedance interconnects at microwave frequencies. It's a transmission line structure used in microwave circuits. Microstrip consists of a conductor trace on top of a dielectric substrate, with a ground plane on the bottom. The trace width, substrate thickness, and dielectric constant determine the characteristic impedance (typically 50 ohms). Microstrip is used extensively in microwave circuits because it can be fabricated on PCBs, provides controlled impedance, and is suitable for high-frequency operation. It's used for interconnects, matching networks, filters, and other microwave components. The precision of the printed conductors and the controlled dielectric provide consistent impedance at microwave frequencies.