In a star, nuclear fusion occurs in the

core of star

Explanation:

When two lighter  nuclie combine to form a heavy nucleus and release a tremendous amount of energy, this reaction is called nuclear fusion. The energy coming from the sun is due to the of nuclear fusion reaction.

The nuclear fusion reaction takes place at very high temperature. Such high temperature is always in the core.

In nuclear fusion, two or more small nuclei combine to form a single larger nucleus, a neutron, and a tremendous amount of energy. Nuclear fusion of hydrogen to form helium occurs naturally in the sun and other stars. It takes place only at extremely high temperatures.