Final answer:
The Soviet economy was ultimately strained by the arms race with the United States, which necessitated heavy military spending, and the economic reforms and political openness under Gorbachev's policies of 'perestroika' and 'glasnost,' leading to the collapse of the Soviet Union.
Step-by-step explanation:
What strained the Soviet economy and played a role in the collapse of the Soviet Union was C) the arms race with the United States. This prolonged period of military competition significantly drained the Soviet finances as the country struggled to keep up with the technological advancements and military expenditures of the United States. The arms race escalated with U.S. President Ronald Reagan's military buildup and Strategic Defense Initiative, which pressurized the Soviets to increase their own spending in defense and research.
Additionally, the Soviet policy under Mikhail Gorbachev known as perestroika, which aimed to reform the command economy, ended up weakening the economic structure further. Gorbachev's glasnost also led to increased political openness but inadvertently encouraged nationalist movements within the USSR, contributing to its disintegration. As the Soviet influence in Eastern Europe weakened, republics began to declare independence, signaling the final collapse of the Soviet Union in 1991.