1. As far as I remember, the Soviet Union established buffer states around its borders b. to protect itself from direct invasion. The most important states was Ukrainian SSR and eastern SSR. To protect borders from invasions from Europe and Asia.
2. Iron Curtain is a. a term for the divide that isolated the East. This division began at the end of WW2 (1945) until the fall of the Soviet Union.
3. I think there could be more than one option. I suggest you to choose between b. They had no access to Western culture. or d. They could not travel freely. As they're both correct. Try your luck.