Question

In what part of the sun's interior do nuclear reactions occur?

Answer 1

The "Core".

Explanation:

The power generator in the Sun is in its center, buried deeply within it. It is called "the core", with a radius close to one fourth of that of the star.

In the core, pressures and temperatures are high enough to force fusion, that is, nuclear reactions whereby some nuclei merge to make others. It is the type of reaction that powers a hydrogen bomb. The most important reaction within the core of the Sun is the process called the "proton-proton cycle."
http://earthguide.ucsd.edu/virtualmuseum/ita/07_2.shtml

Graphics, animations and further discussion:
http://atropos.as.arizona.edu/aiz/teaching/a250/pp.html