Answer: The reason the sun's hot plasma doesn't simply expand into space and cause the sun to fly apart is primarily due to:
c) Gravitational forces
The sun is held together by its own immense gravitational force. The gravitational attraction between the mass of the sun's core and the rest of its material counteracts the outward pressure generated by the high temperatures and pressures resulting from nuclear fusion reactions in the core. This gravitational force prevents the sun from disintegrating and maintains its stable, spherical shape.
While magnetic fields and solar flares play a role in various solar phenomena and processes, it's the overwhelming gravitational force that is the dominant factor in keeping the sun from expanding into space. The nuclear fusion reactions (option d) are the source of the sun's energy, but without the gravitational force, the sun's outer layers would indeed disperse into space.