If two sailing vessels are running free with the wind on the same side, which one must keep clear of the other?

Answer: C — The one to windward

When two sailing vessels are running free with the wind on the same side, the vessel that is to windward must keep clear of the vessel that is to leeward. This stems from the rules of sailing navigation, which dictate that the windward vessel is in a position that generally allows it to take advantage of the wind and is therefore required to give way to the leeward vessel, which may be moving faster due to being downwind. In this scenario, the vessel to windward has a duty to avoid the leeward vessel. The leeward vessel is the one that is situated further away from the source of the wind; as such, it has the right-of-way and the windward vessel must take actions to keep clear to avoid a collision. This principle is fundamental in ensuring safe navigation for sailing vessels under these conditions. Understanding the dynamics of wind direction and vessel positioning is crucial for safe sailing practices.