Final answer:
Alan Turing was the scientist mainly responsible for building the Enigma decoding machine. His work on the machine played a crucial role in cracking the German Enigma code during World War II.
Step-by-step explanation:
Alan Turing was the scientist mainly responsible for building the Enigma decoding machine. He was a British mathematician and computer scientist who played a crucial role in cracking the German Enigma code during World War II. Turing's work on the Enigma machine helped the Allies gain valuable intelligence and played a significant role in shortening the war.