The correct answer is B) Navigate as though the effective range of the radar has been reduced. In heavy rain, the radar signals are attenuated and scattered by the precipitation, which can significantly reduce the effective range of the radar system. Therefore, when using the ARPA (Automatic Radar Plotting Aid) in heavy rain, the appropriate action is to navigate as if the radar's range has been reduced, rather than trying to compensate by increasing the gain or adjusting other settings. The other options are incorrect because: A) Increasing the radar gain may introduce more noise and clutter, making it harder to distinguish real targets; C) Increasing the guard rings may not effectively compensate for the reduced range; and D) Increasing the STC (Sensitivity Time Control) setting is more appropriate for reducing close-in spurious signals, not for addressing the effects of heavy rain.