Which of the following is NOT one of the United States Armed Forces?

The United States Armed Forces comprise several distinct branches, including the Army, Navy, Air Force, Marine Corps, and Coast Guard. The United States Army is responsible for ground-based military operations, the Navy conducts naval operations and ensures maritime dominance, and the Air Force focuses on aerial warfare, air defense, and air support.

The term "United States Frontier Force" is not recognized as one of the branches of the U.S. Armed Forces. It does not exist as an official or recognized military branch, which makes it the correct answer to the question asking for which option is not part of the armed forces. While there have been historical entities and discussions around frontier protection and forces, none of these have culminated in a formal branch of the military. This lack of recognition is what sets "United States Frontier Force" apart from the well-established branches of the U.S. Armed Forces.