c. core
Explanation:
The core is the most inner part of a star, and it is the region where the fusion of the hydrogen occurs.
In the fusion of the hydrogen, two nuclei of hydrogen come very close to each other (overcoming the electrostatic repulsion thanks to the very high temperature inside the star) and combine together forming a nucleus of helium. In the reaction, the mass of the two initial nuclei of hydrogen is smaller than the mass of the final nucleus, so some mass has been converted into energy according to Einstein's equation
[tex]E=mc^2[/tex]
In this process, therefore, a huge amount of energy is released, and this is the energy that keeps the star alive: in fact, without this energy that pushes "outward", the star would collapse as a result of the huge gravitational attraction that it exerts on itself.
