What publication mandated the reassignment of black Airmen to previously all-white units in 1949?

The correct answer is based on the historical context of racial integration in the U.S. military. Air Force Letter 35.3 was significant in directing the integration of black Airmen into all-white units, marking an important step towards equality and civil rights within the military. This letter was part of the broader effort to desegregate the armed forces, which began following President Truman's 1948 Executive Order 9981 that mandated equality of treatment and opportunity in the military.

The other options, while related to various Air Force policies and regulations, do not pertain to the specific mandate for the reassignment of black Airmen to previously all-white units. Thus, Air Force Letter 35.3 stands out as the publication that directly addressed this crucial change in military policy during that era. This historical moment not only impacted the composition of military units but also reflected broader social changes regarding race relations in the United States.