The correct answer is D) GM. GM (Metacentric Height) is used as an indicator of a vessel's initial stability. GM is the vertical distance between the center of gravity (G) and the metacenter (M). A larger GM value indicates greater initial stability, as it makes the vessel more resistant to heeling or rolling. This is an important parameter that is evaluated during the US Coast Guard Captain's License Examinations. The other options are not directly used as indicators of initial stability. KG (Vertical Center of Gravity) and KM (Vertical Center of Buoyancy) are related parameters, but GM is the specific measure that indicates a vessel's initial stability.