Which insignia represents a Navy or Marine Corps O-6?

The insignia representing a Navy or Marine Corps O-6 is an eagle. In the rank structure of the United States Navy and Marine Corps, the O-6 rank corresponds to Captain in the Navy and Colonel in the Marine Corps. The eagle insignia specifically denotes this rank, symbolizing authority and leadership within the armed forces.

In contrast, the other options represent different ranks: a silver star is used for the O-7 rank, which is a Rear Admiral (Lower Half) in the Navy, while two silver stars denote a Rear Admiral (Upper Half) at the O-8 rank. Three gold stars signify a Lieutenant General in the Marine Corps or a Vice Admiral in the Navy. Therefore, the eagle is uniquely associated with the O-6 rank, making it definitively the correct answer.