When is a stand-on vessel first allowed by the rules to take action in order to avoid a collision?

A) When a collision is imminent.
B) The stand-on vessel is never allowed to take action.
C) When the two vessels are less than half a mile from each other.
D) When the give-way vessel is not taking appropriate action to avoid a collision.

The stand-on vessel is allowed to take action to avoid collision when the give-way vessel is not taking appropriate action to avoid collision, as per Rule 17 of the International Regulations for Preventing Collisions at Sea (COLREGs).

Explanation:

A stand-on vessel refers to a boat that has the right-of-way according to navigational rules and is expected to maintain its course and speed. The rules of the road at sea are covered by the International Regulations for Preventing Collisions at Sea (COLREGs). According to these rules, the stand-on vessel should maintain its course and speed to allow the give-way vessel to take action and avoid a collision. However, when the give-way vessel is not taking appropriate action to avoid collision, the stand-on vessel is then allowed, and indeed required, to take action to avoid an accident.

The specific rule that describes this situation is Rule 17 of the COLREGs which states that the stand-on vessel may take action to avoid collision by its maneuver alone as soon as it becomes apparent that the give-way vessel is not taking adequate measures to comply with the rules. This does not necessarily relate to a specific distance; rather, it relates to the situation at hand and the effectiveness of the expected actions of the give-way vessel.