Final answer:
The Soviet Union (USSR) was the former enemy of Hitler who signed the nonaggression Molotov-Ribbentrop Pact, or Nazi-Soviet Pact, with Germany in 1939, allowing for a coordinated invasion of Poland. Hence, option a) is the correct answer.
Step-by-step explanation:
The former enemy of Hitler who signed a nonaggression pact with Germany in 1939 was the Soviet Union (USSR). This pact, also known as the Molotov-Ribbentrop Pact or the Nazi-Soviet Pact, was signed on August 23, 1939, by Nazi Germany and the Soviet Union. The agreement promised that both countries would not attack each other and remain neutral in case of a war involving the other. It allowed for a coordinated invasion of Poland in September 1939, with Germany attacking from the west and the Soviet Union from the east. This secret protocol also included the division of Eastern Europe between the two nations.