The correct answer is option 2: USA and Russia. The START (Strategic Arms Reduction Treaty) is a treaty between the United States and Russia. This treaty is an important arms control agreement that aims to reduce and limit the number of strategic nuclear weapons possessed by both countries. It was signed in 1991 and entered into force in 1994. The treaty was later replaced by the New START Treaty in 2011, which also includes further arms reductions and increased transparency measures.

Option 1, China and Japan, is incorrect. While both countries are significant players in global politics and have their own agreements and relations, the START treaty does not involve them.

Option 3, USA and the European Union, is also incorrect. The European Union is not a sovereign state and does not have the authority to negotiate international treaties. While the US and EU have a strong alliance and cooperate on various issues, the START treaty specifically involves Russia, not the European Union.

Option 4, China, India, USA, and Russia, is also incorrect. While China and Russia are involved in the START treaty with the United States, India is not a party to this particular treaty. Therefore, the correct answer remains option 2: USA and Russia.