Explanation: Range markers in radar and similar ranging systems are used to indicate specific distances from the transmitter. The fundamental principle of these systems is measuring the time it takes for a transmitted pulse to travel to a target and return. Therefore, determining range involves precise time measurement. A **timer** (a timing circuit) is essential for generating these markers. It starts counting from the moment a pulse is transmitted and, at predetermined time intervals, generates a marker. Since distance equals speed (of light) multiplied by time, these timed markers directly correspond to calibrated distances displayed on the screen. A) The CRT (Cathode Ray Tube) is merely the display device; it shows the markers but doesn't create or time them. B) The magnetron generates the high-power radio frequency transmit pulse, but it doesn't determine the timing for the range markers after the pulse is sent. D) The video amplifier processes the received echo signals from targets, not the internally generated range markers.