When are vessels required to sound a fog signal under Rule 35?

Answer: B — When they are constrained by visibility or at a standstill

Vessels are required to sound a fog signal under Rule 35 when they are constrained by visibility or at a standstill. This rule is particularly important in conditions of reduced visibility, such as fog, rain, or snow, where the ability of vessels to see each other is limited. The purpose of sounding a fog signal is to alert other vessels of one's presence to prevent collisions. In instances where a vessel is at a standstill, it is equally important to signify its location and avoid any unintended accidents. This requirement ensures that all vessels are aware of each other's positions, thus enhancing safety on the water during challenging visibility conditions. The other options do not accurately reflect the conditions under which fog signals are mandated. For instance, signaling only in crowded waterways or exclusively at night does not encompass all scenarios requiring caution in reduced visibility. Additionally, simply entering a harbor does not inherently indicate the need for a fog signal unless visibility is compromised, making the requirement under Rule 35 more specific to visibility and the vessel's operational status rather than the general conditions mentioned in the other choices.