The correct option is Heat.
Explanation
Evaporation is the process by which water is transformed from a liquid to a gaseous state. This phase change is fundamentally driven by energy input.
- Heat: Heat is the primary cause of evaporation. The transformation requires energy to break the intermolecular forces holding water molecules in the liquid phase. This energy is referred to as the latent heat of vaporisation. An increase in temperature directly increases the kinetic energy of water molecules, facilitating their escape into the atmosphere as vapour.
- Pressure: An increase in atmospheric pressure generally opposes evaporation by exerting force on the liquid surface, making it more difficult for molecules to escape.
- Dust Particles: Dust particles act as hygroscopic nuclei which are essential for condensation (the conversion of vapour back to liquid), not evaporation.
- Wind Speed: While wind speed affects the rate of evaporation (greater wind speed replaces saturated air with drier air, enhancing the rate), a decrease in wind speed slows the process and is not the cause of the phase change itself.
Key Takeaway: Heat is the essential energy source required to overcome the latent heat of vaporisation, enabling the phase transition of water from liquid to gas.