Explanation: A SART (Search and Rescue Transponder) operates by detecting radar pulses from a searching vessel's X-band radar. Upon detecting a radar pulse, the SART transmits a series of 12 equally spaced responses. On the radar's PPI (Plan Position Indicator) screen, these responses typically appear as a line of 12 distinct dots extending radially outwards from the SART's position. As the searching vessel approaches the SART, the strength of the radar's interrogation pulse and the SART's response increases significantly. This causes the individual dots on the PPI display to merge together. Initially, they form arcs, and when the rescue vessel is in the immediate vicinity, very close to the SART, these arcs will merge further to become a series of concentric circles centered around the SART's exact location. This unique and dynamic display is the definitive visual cue for rescue personnel to pinpoint a SART in close proximity. Option A is incorrect because DSC (Digital Selective Calling) units operate on VHF/HF frequencies and are not designed to detect the X-band radar signals emitted by a SART. Option B is incorrect. While SARTs aid in locating survivors, they provide a relative position on a radar screen, not an absolute GPS-like coordinate with a specified accuracy like 2 nautical miles; that figure is more commonly associated with older EPIRB (Emergency Position-Indicating Radiobeacon) accuracy. Option C is incorrect. The SART's signal has a very specific appearance (dots to arcs to circles) when detected by radar; it does not merely "come and go" due to swells, although extreme conditions can affect any radar return. The characteristic change in display is the key indicator.