When is a head-on situation deemed to exist?

A head-on situation is considered to exist when two vessels are approaching each other and each sees the other nearly ahead. This situation occurs regardless of the time of day, meaning it is not limited to night conditions. It typically involves both vessels being on a collision course, which necessitates action to avoid a collision.

In the context of the COLREGs, vessels in a head-on situation must alter course and speed to ensure safe passing. The key factor here is the visibility of the other vessel directly ahead, making option B the most appropriate answer.

Other scenarios, such as vessels being parallel, do not define a head-on encounter. Similarly, it is not relevant whether only one vessel is under power or if the situation occurs during the day or night. The crucial aspect is the relative position of the vessels, which defines the immediate need for collision avoidance actions.