Explanation: The Variable Range Marker (VRM) system is used in radar and other ranging applications to precisely measure the distance to a target. Its fundamental function is to generate a single, adjustable range ring (or strobe line) on the display. An operator can then align this adjustable ring with a target of interest, and the system provides a digital readout of that target's range. * **B) The VRM system develops a single adjustable range ring.** This is correct. The "variable" aspect means the operator can move this single ring to any desired range for measurement. * **A) The VRM is an auxiliary output of the fixed range marker oscillator.** This is incorrect. Fixed range markers (FRMs) provide multiple, non-adjustable range rings at set intervals. While both VRM and FRM systems use timing circuits, the VRM is an independent, user-controlled system, not just an auxiliary output of a fixed system. * **C) The VRM system is calibrated using a frequency counter.** This is incorrect. While internal oscillators might be checked with a frequency counter, the primary calibration of a ranging system involves correlating timing delays to known physical distances, often using test targets at measured ranges or precise time interval generators. * **D) The VRM system is controlled by a crystal oscillator.** This is incorrect. A crystal oscillator provides a highly stable *fixed* frequency. For a range marker to be *variable*, its timing must be adjustable, which requires additional circuitry beyond a simple crystal oscillator. The variability comes from a user-controlled timing element, not a fixed-frequency crystal.