Here are the correct answers to your questions:
Question 1: Who was the eighth and final leader of the Soviet Union during its collapse?
Answer: Mikhail Gorbachev
Question 2: What led to the decline of the Soviet Union's economy?
Answer: Being lagging behind the Western world
Question 3: Which two reforms did Gorbachev introduce to try and revitalize the Soviet economy?
Answer: Perestroika and glasnost
Question 4: What significant event happened on November 9, 1989, signaling Soviet leadership failures?
Answer: Destruction of the Berlin Wall