Hydrogen fusion occurs in the of the sun.

photosphere
corona
core
radiative zone
chromosphere
convective zone

core

Explanation:  Nuclear fusion is a reaction in which two or more than two nuclei fuse together to form a larger and stable nuclei.

Thus in order to bring two nuclei close, it requires a lot of energy to overcome repulsive forces between them. This can be achieved by supplying large amount of energy and this is present in the core of the sun which is at high temperatures.

The other parts of sun called photosphere, corona, radiative zone, chromosphere and conductive zone have relatively low temperatures and thus fusion cannot take place here.

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