Under Rule 35, which condition requires vessels to sound a fog signal?

Answer: B — Restricted visibility or when at a standstill

Rule 35 of the COLREGs specifically addresses the actions vessels must take in restricted visibility, which is the primary condition that necessitates sounding a fog signal. This rule aims to prevent collisions and ensure safety when visibility is limited due to fog, rain, or other atmospheric conditions that obstruct a vessel's line of sight. In situations of restricted visibility, vessels are required to sound specific signals to indicate their presence to other vessels nearby. These signals help mariners determine the location and movement of vessels around them, which is critical for navigating safely in such conditions. While high traffic areas, the size of the vessel, or stormy weather can contribute to navigational challenges, they do not specifically mandate the sounding of fog signals as outlined in Rule 35. Thus, restricted visibility is the key condition prompting this safety requirement.