The process involved in the formation of a crust on rocks due to the vapours of sulphur escaping from a volcano is an example of deposition. Deposition is a phase change process where a substance changes from a gaseous state to a solid state directly, without going through the liquid state.

Option 1: Condensation is the process of a gas changing into a liquid state. It occurs when the gas loses thermal energy and the particles come together to form liquid droplets. This does not apply to the given scenario as the sulphur vapors are directly solidifying on the rocks.

Option 2: Precipitation refers to the process of water droplets falling from the atmosphere to the ground as rain, snow, sleet, or hail. This option is not applicable to the given scenario as it specifically deals with water, not sulphur vapors.

Option 4: Evaporation is the process by which a liquid changes into a gas state. It does not correspond to the given scenario as the vapors are not turning into a gas form.

In conclusion, option 3 - deposition is the correct answer as it accurately describes the process of the vapors of sulphur directly solidifying and forming a crust on the rocks from the volcano.