Answer : 4. "Japan"
Explanation :

Answer: D) Japan Explanation: Maglev trains are used in Japan, South Korea and China only. Maglev derived from magnetic levitation using the principle of magnetic repulsion is a system of train transportation that uses two sets of magnets, one set to repel and push the train up off the track as in levitation then another set to move the 'floating train' ahead at great speed taking advantage of no friction. Maglev trains are therefore quieter and smoother than conventional trains, and have the potential for much higher speeds Despite over a century of research and development, maglev transport systems are in operation in just three countries Japan, South Korea and China.   Hence Maglev -- Magnetic-levitation.