Which insignia represents the rank just below Captain?

The rank just below Captain in the naval rank structure is Commander. This position is often associated with a significant level of responsibility, typically commanding a warship, submarine, or an aviation squadron. Commanders play a crucial role in overseeing operations and ensuring mission readiness and effectiveness within their units.

While the other options represent various ranks and positions within the Navy, they do not fall directly in the rank hierarchy below Captain. For example, a Commanding Officer is not a specific rank but refers to the officer in charge of a unit, and it could be a Captain or a Commander depending on the size of the command. An Ensign is an entry-level rank for officers, much lower than Commander, while Chief Warrant Officer 4 is a specialized rank above enlisted personnel but also below Commander. Thus, Commander stands out as the correct answer, accurately representing the rank directly beneath Captain.