What action is a power-driven vessel required to take when encountering a sailing vessel on a collision course on her port side?

Answer: A — Keep clear, passing at a safe distance.

A power-driven vessel encountering a sailing vessel on a collision course to its port side must keep clear, passing at a safe distance. This is rooted in the navigational rules that prioritize the responsibilities of different types of vessels. According to the Rules of the Road, when a power-driven vessel meets a sailing vessel with the wind on the latter's port side, the power-driven vessel is considered the give-way vessel. It is imperative for the power-driven vessel to take early and substantial action to avoid a collision. Keeping clear ensures that the sailing vessel, which typically has limited maneuverability compared to a power-driven vessel, can continue its course without danger. This is essential to maintain safety on the water and prevent accidents. The other options do not align with maritime navigation rules in this scenario. For instance, altering course to starboard could potentially place the power-driven vessel in harm's way, while sounding a danger signal may only be necessary if there is an imminent risk of collision but does not serve as a preventative action. Maintaining course could lead to a collision, which is contrary to the goal of safe navigation.