The correct answer is B) Risk of collision may exist when the compass bearing of an approaching vessel is changing appreciably. This is correct because the relative motion between two vessels is a key factor in determining the risk of collision. When the compass bearing of an approaching vessel changes appreciably, it indicates that the relative position of the vessels is changing, which could lead to a collision risk. However, a changing bearing does not always guarantee a collision risk, as other factors such as speed and distance must also be considered. Option A is incorrect because the stand-on vessel is required to maintain course and speed, while the give-way vessel must take action to avoid a collision. Option C is incorrect because the risk of collision does not always exist when vessels pass within one mile of each other, as it depends on their relative motion and other factors. Option D is incorrect because a changing compass bearing alone does not necessarily mean that a collision risk always exists.