Explanation: On a ship equipped with a radio direction finder (RDF), an auxiliary receiving antenna serves a crucial safety function. The RDF typically uses a specialized antenna, often a directional loop, which may be connected to its own receiver. When the RDF is actively being used to take a bearing, its antenna system is engaged. The auxiliary receiving antenna provides a separate, independent receive path for the ship's main communications receiver. This setup ensures that the radio operator can continuously monitor critical frequencies, such as the international radiotelegraph distress and calling frequency of 500 kHz, regardless of whether the RDF system is currently in use for navigation or homing. This maintains the essential safety watch at sea. Option A is incorrect because its primary purpose isn't just a spare, but to enable simultaneous, independent operation. Option C is incorrect because the auxiliary antenna is for the communications receiver to operate independently of the RDF, not the other way around.