Explanation: ARPA (Automatic Radar Plotting Aid) and CAS (Collision Avoidance System) refer to advanced features integrated with marine radar systems. Their primary purpose is to assist navigators in preventing collisions by automating the process of target tracking. These systems automatically **acquire and track targets** detected by the radar, calculating critical information such as the target's course, speed, closest point of approach (CPA), and time to CPA (TCPA). This processed data is then displayed visually, typically as vectors or symbols, on the radar indicator's CRT (or modern LCD screen), enabling the operator to quickly assess collision risks and make informed decisions. Option A is incorrect because ARPA/CAS is an advanced *enhancement* to a basic radar system, not the basic system itself. Option B describes only a display function and is overly specific; ARPA/CAS's core is the processing and tracking. Option D is incorrect because ARPA/CAS provides crucial decision-making information to the human operator but does not automatically steer the vessel.