Before rainfall forms, which process involving water vapour is important in clouds?

Correct answer: Condensation

Clouds form when moist air rises, cools, and reaches conditions in which water vapour can change into liquid water. This change from invisible vapour into tiny liquid droplets is called condensation. The droplets collect around very small particles in the air and make clouds visible. As droplets collide and join, they may become large and heavy enough to fall toward the ground as rain. Evaporation supplies vapour earlier in the water cycle, but condensation is the important cloud process immediately connected with droplet formation.

Option D is correct because condensation changes water vapour into cloud droplets, which can grow and produce rainfall. Option A is not the best answer: evaporation changes liquid water into vapour and generally supplies moisture before cloud formation. Sedimentation and erosion do not create rain droplets in clouds. Thus, the essential process inside a rain-producing cloud is condensation, followed by droplet growth and falling precipitation.