The correct answer is D) GM. Initial stability is indicated by the metacentric height (GM), which is the vertical distance between the center of gravity (G) and the metacenter (M). A positive GM value indicates the vessel has initial stability and will tend to right itself when disturbed from an upright position. The larger the GM value, the more stable the vessel will be. The other answer choices are incorrect because: A) KM is the vertical distance between the keel and the metacenter, which is not a direct indicator of initial stability. B) Deck load does not directly indicate initial stability. C) Maximum allowed KG is a limitation on the vertical position of the center of gravity, not a measure of initial stability.