The correct option is Cohesion.
Explanation
The formation of the solar system is explained by the Nebular Hypothesis, which posits that the Sun and planets formed from a rotating cloud of gas and dust. The transition from microscopic dust grains to larger solid bodies is the first step in planetary formation.
Detailed Analysis:
- Cohesion (Correct): As the solar nebula cooled, gas condensed into solid dust grains. These grains collided with one another. The primary physical process allowing these small particles to stick together and grow into larger clumps is cohesion (often driven by electrostatic forces and molecular bonds). This process of aggregation, known as accretion, eventually leads to the formation of planetesimals (small celestial bodies).
- Fission (Incorrect): Fission involves the splitting of a body into two or more parts. While the fission theory was once proposed for the origin of the Moon, it does not explain the aggregation of dust into planetesimals.
- Explosion (Incorrect): An explosion involves the rapid expansion and scattering of material, which is contrary to the aggregation process required to form solid bodies.
- Evaporation (Incorrect): Evaporation refers to the phase change from liquid or solid to gas. In the early solar nebula, evaporation would result in the loss of solid material, not its accumulation.
Key Takeaway: Planetesimals are formed through the process of accretion, initiated by the cohesion of dust grains in the solar nebula, which allows matter to accumulate into larger solid bodies.