The correct answer is D) Warn of small targets that are initially detected closer than the outer guard zone. The inner guard zone on an ARPA (Automatic Radar Plotting Aid) system is designed to detect and alert the watch officer of small targets that are initially detected closer than the outer guard zone. This allows the watch officer to be aware of potential hazards, such as small vessels or debris, that may not have been picked up by the outer guard zone. The inner guard zone provides an additional layer of protection and early warning for the navigation of the vessel. The other answer choices are incorrect because: A) is not the primary purpose of the inner guard zone, B) refers to the outer guard zone's function, and C) is a more general purpose of ARPA systems, not specifically the inner guard zone.