What is ‘Tokamak’ ?

A. An Israeli radar system
B. An American anti-missile system
C. A defense collaboration between Japan and South Korea
D. A device used in nuclear-fusion research for magnetic confinement of plasma

Explanation:

A tokamak is a device that uses a powerful magnetic field to confine plasma in the shape of a torus. The tokamak is one of several types of magnetic confinement devices being developed to produce controlled thermonuclear fusion power. As of 2016, it was the leading candidate for a practical fusion reactor.