What must vessels indicate once the risk of collision exists?

Answer: B — Indicate maneuvers using sound signals

When the risk of collision exists, vessels are required to indicate their intentions and maneuvers using sound signals. This is crucial for effective communication between vessels to ensure safety and prevent misunderstandings at sea. The use of sound signals allows other vessels to understand the actions a vessel is about to take, which is especially important in conditions of reduced visibility or when vessels are maneuvering in proximity to one another. In situations where a collision risk is present, it is imperative that vessels alert others of their intended movements through appropriate sound signals as outlined in the COLREGs. This fosters situational awareness and promotes a coordinated response to the evolving circumstances, enhancing overall navigational safety.