The atomic bomb dropped by the USA on the Japanese city of Hiroshima on 6 August 1945 was code-named:

The atomic bomb dropped by the United States on Hiroshima on 6 August 1945 was code-named "Little Boy"; it was the first nuclear weapon ever used in war.

About Little Boy:

• It was a uranium bomb, using uranium-235, and worked on the "gun-type" design, in which one piece of uranium was fired into another to start the chain reaction.
• It was dropped from the B-29 bomber "Enola Gay", piloted by Colonel Paul Tibbets.
• It exploded about 600 metres above the city, with a force equal to about 15,000 tonnes of TNT.
• About 70,000 to 80,000 people were killed instantly, and many more died later from injuries and radiation sickness.

The second bomb:

• On 9 August 1945, a second atomic bomb, code-named "Fat Man", was dropped on Nagasaki.
• Fat Man was a plutonium bomb, using the "implosion" design.
• About 40,000 people were killed instantly at Nagasaki.

The end of the war:

Japan announced its surrender on 15 August 1945, and signed the formal instrument of surrender on 2 September 1945, ending the Second World War.

The Manhattan Project:

• The atomic bombs were developed under the secret "Manhattan Project" of the USA.
• Robert Oppenheimer was the scientific director of the Los Alamos laboratory, where the bombs were designed.
• The first test explosion, code-named "Trinity", was carried out in the desert of the south-western USA on 16 July 1945.

Remembrance:

• 6 August is observed as Hiroshima Day.
• The Hiroshima Peace Memorial (Genbaku Dome) is a UNESCO World Heritage Site.

After the war:

• The bombings led to a nuclear arms race between the USA and the Soviet Union.
• The Treaty on the Non-Proliferation of Nuclear Weapons (NPT) was signed in 1968.
• India conducted its first nuclear test at Pokhran in 1974 and further tests in 1998.

Why not the other options?

• ✗Fat Man was the bomb dropped on Nagasaki; Trinity was the first test; Thin Man was an earlier design that was abandoned.