The "manhattan project" was the code name for the a) u. s. plan to invade japan. b) german use of jet aircraft. c) allied plan to invade europe. d) u. s. plans to make an atomic bomb.

In the United States, the "Manhattan Project" was the codename for the "D) U.S. plans to make an atomic bomb," since this was an incredibly sensitive and dangerous project that was held under the highest levels of secrecy. 

The "Manhattan Project" was the code name for the U.S.plans to make an atomic bomb.