Explanation: RADAR collision avoidance systems, especially Automatic Radar Plotting Aids (ARPA), integrate various sensor inputs to accurately track targets, calculate their motion, and predict potential collision situations. An **anemometer (C)** measures wind speed and direction. This meteorological data is useful for navigation planning related to weather or ship performance, but it provides no direct input for calculating the relative motion of other vessels, which is the core function of a collision avoidance system. In contrast, a **gyrocompass (A)** provides your ship's accurate true heading, which is crucial for stabilizing the radar display, calculating true bearings of targets, and determining their true course and speed. A **navigation position receiver (B)**, such as a GPS, supplies your ship's precise geographical position and often its speed over ground, enhancing overall situational awareness and supporting true motion plotting. Your ship's **speed indicator (D)**, or speed log, is vital for determining your vessel's own motion vectors, enabling the ARPA system to accurately compute target vectors, Closest Point of Approach (CPA), and Time to Closest Point of Approach (TCPA). These three inputs are essential for a functional and reliable collision avoidance system.