Which country was considered to be the successor country of the Soviet Union with regard to its military and position in the UN Security Council?

Russia

The correct answer is Option 1 - Russia

The Soviet Union disintegrated in 1991. Russia was now accepted as the successor state of the Soviet Union. It inherited the Soviet seat in the UN Security Council. Russia accepted all the international treaties and commitments of the Soviet Union. It took over as the only nuclear state of the post-Soviet space and carried out some nuclear disarmament measures with the US.