What action must the stand-on vessel take when a risk of collision exists?

Answer: C — Maintain course and speed

The stand-on vessel has the responsibility to maintain its course and speed when there is a risk of collision with another vessel, as outlined in the COLREGs. This rule is in place to promote safety at sea by preventing confusion with abrupt maneuvers. By maintaining its course and speed, the stand-on vessel provides predictability in its navigational behavior, allowing the give-way vessel to effectively assess the situation and take action to avoid a collision. When a risk of collision is identified, the stand-on vessel may still need to monitor the situation closely. If the give-way vessel does not take adequate measures to avoid a collision, the stand-on vessel is allowed to take measures as necessary to ensure safety. However, the initial action required is to continue on its established path rather than initiating maneuvering actions that could lead to further risk or miscommunication between the vessels involved. Changing course immediately, reducing speed significantly, or preparing to anchor are not the primary obligations of the stand-on vessel under these circumstances and could potentially contribute to confusion or risk if the give-way vessel is not aware of these changes. Therefore, maintaining course and speed is essential to both ensure safety and comply with the COLREGs.