All the listed factors contributed to the struggles of the Soviet economy in the 1980s, but the most significant was high military spending. This was part of the arms race during the Cold War and diverted resources away from other vital sectors, such as consumer goods and agricultural production, exacerbating the economic issues.
However, lower oil prices also had a considerable impact because the Soviet economy was heavily reliant on oil exports for revenue. A drop in oil prices in the 1980s significantly reduced state income.
In an academic sense, if forced to choose one primary factor that hurt the economy, high military spending is often highlighted as a major detriment.