Under which condition must a vessel reduce speed according to the rules for restricted visibility?

Answer: D — If navigating in limited visibility conditions

In situations of limited visibility, the COLREGs emphasize that a vessel must take appropriate measures to avoid collisions, which includes reducing speed as necessary. This obligation is grounded in Rule 19, which specifically addresses vessels operating in restricted visibility. When visibility is poor, such as during fog or heavy rain, the ability to see other vessels and navigational markers is significantly compromised. By reducing speed, a vessel enhances its ability to react appropriately to the presence of other vessels or hazards, thereby increasing safety. Although other conditions like having another vessel in sight or navigating in congested waters might suggest a need for caution, the primary requirement to reduce speed in the context of restricted visibility is clearly articulated in the rules. The goal is to maintain control and ensure the vessel can respond to unexpected situations as they arise.