Final answer:
To avoid a collision, both vessels should alter course to starboard, moving to their right, as this is a standardized maritime rule that ensures clear and predictable actions for safety.
Step-by-step explanation:
When two vessels are approaching each other head-on, or nearly so, the action to avoid a collision, according to international maritime rules, is 'C. Both vessels should alter course to starboard'. This means that both the ships should move to their right. By doing this, the paths of the vessels diverge, minimizing the risk of a collision.
It is important to note that the decision on avoiding collisions does not depend on who sights the other vessel first or the speed at which either vessel is moving. The rule is standardized to ensure that when in doubt, both parties have a clear and predictable action to follow that increases the safety of both vessels.