The correct answer is Enola Gay, the B-29 Superfortress bomber that dropped the atomic bomb 'Little Boy' on Hiroshima on August 6, 1945. Option D has a typo ('Inola' instead of 'Enola') but is clearly intended to reference this historical aircraft. The other options are incorrect: Topolov-II is a Soviet bomber, B-2 is a modern stealth bomber, and U-21 is not a nuclear bomber designation.