According to the nebula theory, what is believed to be the origin of the solar system?

The nebula theory posits that the solar system formed from a large cloud of gas and dust in space, known as a nebula. According to this theory, the origin of the solar system began with the gravitational collapse of this nebula. As regions within the nebula experienced increased density, gravity pulled matter together, leading to the collapse of the cloud. This collapse caused the materials within the nebula to clump together, ultimately forming the Sun at the center and the planets, moons, asteroids, and comets in orbit around it.

The central role of gravitational forces in this process is fundamental, as they govern the dynamics of the nebula's collapse. As material coalesced, it not only formed the Sun but also shaped the orbits and structures of the rest of the solar system. This gravitational collapse mechanism is critical to understanding how celestial bodies form and is a key concept in astrophysics.

Other processes mentioned in the incorrect options, such as chemical reactions, collisions of primordial matter, or cooling of solid substances, do not directly address the initial conditions and mechanisms described by the nebula theory, which focuses predominantly on the effects and influence of gravity.