The correct answer is C) large GM. A large GM (metacentric height) indicates the vessel has a high degree of initial stability, which can result in rapid and excessive motion in a seaway. This rapid motion is due to the large righting moment (GZ) that tries to quickly return the vessel to the upright position. In contrast, a small GZ (answer B) would result in slower, more sluggish motions. Excessive free surface (answer A) reduces stability and would not cause rapid motions. A high center of gravity (answer D) reduces stability and can also lead to rapid motions, but a large GM is the primary cause of the quick, excessive motion described in the question.