Final answer:
The Rules of the Road require both vessels to change course when two power-driven vessels are crossing and the give-way vessel is not taking appropriate action to avoid a collision.
Step-by-step explanation:
When the Rules of the Road at sea require both vessels to change course, it is specifically under certain circumstances for the safety of navigation. The situation described in option B applies; that is when two power-driven vessels are crossing, and it is apparent to the stand-on vessel that the give-way vessel is not taking appropriate action. This measure is taken to avoid a collision when the give-way vessel is not yielding as required.