Answer:
Having friendly, pro-communist countries along all their borders.
Step-by-step explanation:
The Soviet Union's main goal after the end of the Second World War was to propagate and help build communism. This will allow them to be on equal ideological beliefs and not hinder or even oppose them. And that will enable them to dominate and have world power over other countries.
So, for them, having neighbors who have the same ideological beliefs was a big part of their desire to feel safe. They believe that with pro-communist neighbors, then they will not be attacked or invaded, thus securing their safety from outside invasions.
Thus, the correct answer is the third option.