Which organizational structure did Security Forces initially fall under?

The correct choice is that Security Forces initially fell under the Army Provost Marshal. This affiliation is significant because the Army developed the military police concept, and the roles of law enforcement and security forces within the military have their roots in the Army's structure and operations.

The Army Provost Marshal provided the foundation for military police functions, which included law enforcement, prison operations, and security operations on military installations. As the Air Force evolved and established its own security forces, the principles and practices derived from the Army's system were adapted to meet the specific needs and operational requirements of the Air Force.

Understanding this relationship provides insight into how Security Forces gained the authority and guidelines that govern their operations today, reflecting the best practices of military law enforcement established initially by the Army.