The correct answer is B) on the navigation bridge. During a "Vessel Grounded Drill", the plan indicating the vessel's watertight boundaries is required to be located on the navigation bridge. This is because the navigation bridge is the primary command and control center of the vessel, where the Captain and officers need to have immediate access to critical information like watertight boundaries in the event of a grounding incident. The other options are incorrect because the engine room, ship's library, and damage control locker are not the designated locations for this critical plan. The navigation bridge is the appropriate place to keep this information readily available for the crew to reference during emergency drills and situations.