Explanation: A variable range marker signal is designed to produce a visible indication, such as a bright dot or line, on a display screen (like a radar scope or oscilloscope) at an adjustable position corresponding to a specific range or time interval. For any signal to be displayed visually on a Cathode Ray Tube (CRT) screen, it must modulate the intensity (brightness) of the electron beam. The video amplifier in a display system is responsible for amplifying these intensity-modulating signals, which include target echoes and marker signals, and feeding them to the control grid or cathode of the CRT. This process ensures the marker is visible on the display. * **A) Sweep amplifier:** Controls the horizontal and/or vertical deflection of the electron beam, creating the trace, not the content of the trace. * **B) Low voltage power supply regulator:** Provides stable operating voltage and is unrelated to signal processing for display. * **D) Range ring oscillator:** Generates fixed range rings, which are different from a single, variable range marker.