The correct answer is:
fusion and gravity
In fact:
- Nuclear fusion: the fusion of lighter nuclei into heavier nuclei release large amounts of energy, and this energy "pushes" outwards the star, balancing the gravitational force
- Gravity: the Sun has a huge mass, so there is a strong gravitational force pushing towards the centre of our star. Without the action of the nuclear fusion, the star would collapse due to the effect of the gravity.