The correct answer is D) the US entry into World War II.
The reason that caused the US Army to take over atomic research in the early 1940s was the US entry into World War II.
The arms race was a true concern for the government of the United States when entering World War II. It already knew the kind of weaponry that Germany had developed and the US government also considered that the Germans had already started to build the atomic bomb.
On December 28, 1942, President Franklin D. Roosevelt gave the authorization to start the Manhattan Project, that was the code name for the developing of the atomic bomb. The scientist Robert Oppenheimer was the leader of the project that was developed in Los Alamos, New Mexico.