Explanation: Radar PPI (Plan Position Indicator) scopes display targets in relation to the antenna's bearing. For the display to accurately reflect the target's direction, the scope's sweep line must continuously synchronize with the physical rotation of the radar antenna. Synchro systems (A) are specifically designed for this purpose. A synchro system typically consists of a synchro transmitter, which is mechanically coupled to the rotating radar antenna. As the antenna turns, the transmitter generates an electrical signal that precisely represents the antenna's angular position. This signal is then sent to a synchro receiver (or control transformer) located within the PPI display unit. The receiver translates this electrical signal back into a mechanical rotation or directly influences the sweep deflection, ensuring the PPI scope's sweep always points in the exact direction the antenna is facing. This provides accurate, continuous remote indication and tracking of angular position. Servo systems (B) are broader feedback control systems that might *utilize* synchros but are not the specific mechanism for *sensing and transmitting* angular position itself. DC positioning motors (C) are merely actuators that move to a position, lacking the inherent feedback and synchronization of a synchro system. Differential amplifiers (D) are electronic components that amplify voltage differences, not direct mechanical angular position sensors.