Hap Arnold's designation as a second lieutenant marked the beginning of his career in which branch of the military?

Hap Arnold's designation as a second lieutenant marked the beginning of his career in the Army Air Service, which was the precursor to the United States Air Force. The Army Air Service was established during World War I, reflecting the growing importance and specialization of aviation in military operations. As a second lieutenant, Arnold was part of the early development of air power in the U.S. military, where he eventually rose through the ranks to become a significant leader in aviation history.

The other branches listed—Naval Aviation, Army Signal Corps, and Army Infantry—did not align with Arnold's initial military career path or his contributions to the development of aerial combat and strategy. His work greatly influenced the way air power was integrated into military operations, culminating in the formation of an independent Air Force after World War II. This highlights his essential role in shaping modern military aviation.