The correct answer is D) A gaff rig is dangerous in a calm wind and large swell. This is the correct answer because a gaff rig, which has a four-sided mainsail, can be dangerous in calm wind and large swell conditions. In these circumstances, the large gaff and boom can become uncontrollable and cause the boat to roll or pitch unexpectedly, increasing the risk of capsizing or other accidents. The other answer choices are incorrect because they do not accurately describe dangerous situations for the specified rig types. A tall, marconi sloop rig is not inherently dangerous to tack, and a square-rigged ship is not particularly dangerous to jibe compared to other rig types. A gaff rig in calm wind alone would not be a dangerous situation.