- The sun is a kind of star
- Nuclear fusion happens in the core of a star
- Therefore, nuclear fusion happens in the core of the sun

- Nuclear fusion happens in the core of the sun
- The core is a kind of layer of the sun
- Therefore, nuclear fusion happens in the core layer of the sun

So, nuclear fusion happens in the core layer of the sun
In which layer of the Sun does nuclear fusion occur?
- The sun is a kind of star 
- The core is a kind of layer of the sun 
- Nuclear fusion happens in the core of a star