The collapse of the Soviet Union was influenced by several factors, but among the options you provided, the most significant would be:
A) Calls for independence from Eastern bloc nations
This factor played a crucial role as many Eastern European countries sought sovereignty and democratic reforms, notably in the late 1980s, leading to the weakening of Soviet control over the region and ultimately contributing to the dissolution of the Soviet Union in 1991.
While B) (the signing of the START agreement) and C) (a weakened Soviet military) are relevant to understanding the context of the Cold War and the USSR’s internal challenges, the immediate factor leading to the USSR's collapse was largely driven by the independence movements in its satellite states. D) (intervention by Western nations) had some impact, but it wasn't a direct cause of the Soviet Union's collapse compared to the internal calls for independence.