The correct answer is C) Flood the area under the leak with water. The reasoning is that in the event of an LNG leak, the primary concern is to prevent the vapor from accumulating and creating a potential fire or explosion hazard. Flooding the area with water helps to quickly dissipate and disperse the vapor, minimizing the risk. This is the recommended first precautionary action according to standard LNG safety protocols. The other options are incorrect because: A) Tightening the flange bolts may not immediately stop the leak, and could put personnel at risk. B) Calling the Coast Guard is a secondary action, after taking immediate steps to mitigate the hazard. D) Stopping the cargo transfer is important, but should be done in conjunction with other safety measures like flooding the area.