The collapse of the Soviet Union can be attributed to a combination of factors, but among the options you provided, A) the Soviet Union put all its resources on building the military is one of the significant reasons. This militarization strained the economy, diverted resources away from consumer goods and services, and led to economic problems.
Additionally, C) the Soviet Union reduced its control of the economy is also relevant because the move towards economic reform (like perestroika) led to weakened centralized control, causing further instability.
While options B and D provide some context, they are less central to the broader explanation of the Soviet Union's collapse. If you had to choose one from your options, A is particularly relevant.