Explanation: The Variable Range Marker (VRM) is an adjustable electronic circle displayed on a radar screen. Its primary function is to allow the operator to precisely measure the distance (range) to a specific target. The operator adjusts the VRM until its circumference touches the desired target's echo on the display, and the corresponding range value is then read from a digital display or scale. This provides a dynamic and accurate method for determining the distance to any object, including moving targets. Option A is correct because it directly describes this function: an accurate means of determining the range of a target, whether stationary or moving. Option B is incorrect because bearing (angular direction) is determined by other radar features, such as an Electronic Bearing Line (EBL) or by observing the target's position relative to the heading marker, not the VRM. Option C is incorrect because while you could measure the range to two targets sequentially and then calculate the difference, the VRM's direct purpose is to indicate the range *from own ship* to *a single target*, not the distance between two arbitrary targets. Option D is incorrect. Fixed range rings are integral to the radar's calibration and are not calibrated by the VRM. The VRM is a measurement tool, not a calibration tool for other display elements.