Which of the following nations adopted an ‘Open Door’ policy and eco nomic reforms in 1978?

The correct option is China.

Step-by-Step Explanation:

1. Historical Context: In December 1978, under the leadership of Deng Xiaoping, the Chinese Communist Party initiated a major shift in its economic policy during the Third Plenary Session of the 11th Central Committee.
2. The "Open Door" Policy: This policy, known as the "reform and opening-up" (Gaige Kaige), marked China's transition from a closed, centrally-planned socialist economy to a market-oriented socialist economy. It opened the nation to foreign investment, encouraged private enterprise, and established Special Economic Zones (SEZs) to boost international trade.
3. Conclusion: Among the options listed, South Korea, the USA, and Japan already had market-oriented economies and did not initiate an "Open Door" policy or transition reforms in 1978. Therefore, China is the nation that adopted these historic reforms in 1978.