Explanation: A rotary joint provides a critical connection in microwave systems where continuous 360-degree rotation is required, such as with radar antennas or satellite tracking systems. It allows a stationary waveguide, carrying the signal from the transmitter/receiver, to connect to a waveguide section that is physically rotating with the antenna array. This prevents the waveguide from twisting, breaking, or experiencing signal loss due to mechanical stress, ensuring reliable signal transfer across the rotating interface. Options A and B describe functions performed by waveguide bends, T-junctions, or waveguide switches, respectively, not rotary joints. Option D, while a rotary joint must maintain internal pressurization, describes a general function for various waveguide components, not the primary, unique purpose of a rotary joint.