The correct answer is C) reduce windward mooring line tensions. In a situation where a vessel needs to deballast to a survival draft due to extreme wind and wave loads, using the available thrusters or propulsion would help reduce the tension on the windward mooring lines. This is because the propulsion system can generate forces to counteract the wind and wave forces acting on the vessel, reducing the stress on the windward lines. The other options are incorrect because maintaining constant mooring line tensions (A) or increasing leeward mooring line tensions (D) would not be the appropriate actions in this scenario, and reducing leeward mooring line tensions (B) would not directly address the issue of the windward lines.