The British nuclear weapons project Niels Bohr joined after reaching Britain in 1943 was Tube Alloys.
Bohr escaped German-occupied Denmark with his wife on 29 September 1943 after learning that the Nazis considered his family Jewish. He reached Scotland on 6 October aboard a de Havilland Mosquito operated by the British Overseas Airways Corporation, and British officials quickly brought him into wartime atomic research.
Tube Alloys was Britain’s secret atomic-bomb programme. Bohr was given an office connected with its nuclear weapons development team, where his scientific reputation and expertise in nuclear physics were especially valuable. The project was later combined with the much larger American effort under the August 1943 Quebec Agreement.
The alternatives describe related people or operations but are not the best answer. The MAUD Committee helped assess the feasibility of an atomic bomb; the Manhattan Project was the American programme; and the Alsos Mission investigated Axis nuclear capabilities. Bohr later served as a consultant to the joint Allied effort.