The correct answer is A) reduce to bare steerageway. The International and Inland Navigation Rules (Rule 6 - Safe Speed) require that when navigating in or near fog, you must proceed at a safe speed, taking into account the prevailing circumstances and conditions. If you hear a fog signal ahead, you must navigate with caution and reduce your speed to the minimum required to maintain steerage, as this allows you to take proper and effective action to avoid collision if the risk of collision exists. The other options are incorrect because: B) stopping the engines may not allow you to maintain steerage and maneuver effectively; C) slowing to less than 2 knots may not be the minimum required speed; and D) beginning a radar plot is an additional action, not the primary action required by the Rules in this situation.