Which flag is flown by naval ships to denote a specific mission?

The Navy Jack is specifically used to indicate a naval ship's involvement in a particular mission, especially in the context of operations or readiness. Its distinctive design, featuring the stars and stripes, serves as a symbol of a ship's identity and its operational status. The Navy Jack is traditionally flown at the fore of the ship and signals the ship's readiness for action, as well as its allegiance to the Navy and the United States. Understanding the purpose of the Navy Jack clarifies its significance in naval protocol; it is crucial for communicating the ship's mission to other vessels and for ceremonial situations.

The other options serve different purposes: the Commissioning Pennant is flown by a ship during its commissioning, the Command Flag indicates the authority of a commanding officer, and the United States Ensign signifies the nationality of the ship. Each of these flags has its own distinct context and meaning, but none serve the specific function of denoting a mission as effectively as the Navy Jack does.