The correct answer is D) small GM. The GM (Metacentric Height) of a vessel is a measure of its stability. A small GM indicates that the vessel has a low center of gravity, which results in a slow and easy motion in a seaway. This is because a small GM provides less righting moment, which leads to the vessel's gentle rolling and pitching in the waves. The other options are incorrect because: A) A low center of gravity does not necessarily indicate a slow and easy motion; it is the small GM that is the key factor. B) A stiff vessel, which has a high GM, would exhibit a more abrupt and less comfortable motion in a seaway. C) A large GZ (Righting Arm) is also associated with a high GM, which would not result in a slow and easy motion.