Which of the following indicates a command's unique identifier?

The command’s unique identifier is indicated by the command's UIC, or Unit Identification Code. The UIC serves as a specific reference number that uniquely identifies a Navy command. This code is critical for ensuring accurate tracking, reporting, and logistics management within the Navy's administrative and operational systems. It differentiates one command from another and is essential for inter-departmental communication and coordination.

Other identifiers, such as the work center serial number, are more specific to a particular sub-unit or operational area within a command and do not serve as a broader identifier for the entire command. The Julian date is a calendar system used for tracking days within a year, not an identifier for a command. Lastly, a letter of designation is a document that may denote specific responsibilities or statuses but does not function as a unique identifier like the UIC does. Thus, the command's UIC is the accurate choice for representing a command's unique identifier.