What is displayed as a mark of authority for boardings, examinations, and seizure of vessels?

The Coast Guard Ensign is the correct answer because it serves as the official flag of the U.S. Coast Guard and is recognized as a mark of authority during operations such as boardings, examinations, and seizures of vessels. When a Coast Guard vessel displays its ensign, it signals that the crew is operating under the authority of the Coast Guard, which is crucial for ensuring compliance with maritime laws and regulations.

The use of the Coast Guard Ensign reflects the service's unique role in enforcing laws on the water, conducting search and rescue operations, and ensuring maritime safety and security. Displaying this flag is a clear indication to vessel operators that the Coast Guard is performing its duties, and it generally necessitates cooperation from the vessels being boarded for inspection.

In contrast, while the National Ensign is the flag of the United States and can represent national authority, it is not specific to the Coast Guard's unique law enforcement missions. The Merchant Flag is associated with commercial shipping, and the Navy Jack is traditionally used by the Navy, neither of which convey the authority of the Coast Guard in maritime law enforcement contexts. This specificity is what makes the Coast Guard Ensign the appropriate mark of authority for the situations described.