The correct answer is D) gravity davit. Gravity davits are the most common type of davit found on merchant vessels today. This is because gravity davits are simple, reliable, and do not require complex mechanical mechanisms to operate. They rely on the force of gravity to lower a lifeboat or life raft into the water, making them a practical and widely-used choice for merchant ships. The other answer choices are less common or incorrect. Sheath-screw davits and quadrantal davits are less frequently used designs, while radial davits are more commonly found on naval vessels rather than merchant ships.