Final answer:
The phenomenon where water vapor from humidified air turns into water droplets on a mirror is called condensation. It happens when humid air cools and the water vapor can no longer remain gaseous, leading to the formation of liquid water droplets on the cooler mirror surface.
Step-by-step explanation:
The true description for the situation where water vapor in the humidified air strikes the mirror and turns into a water droplet is an example of condensation. This phenomenon occurs due to relative humidity, which tells us how much water vapor the air contains relative to the maximum possible. When warm, humid air encounters the colder surface of the mirror, the air is cooled below its dew point, and water vapor condenses into droplets. This is also why droplets form on other surfaces, like a cold beverage glass or appear as dew on a spider web. Drops of water on a newly waxed car are another example, where the water forms beads due to a lack of strong interaction between the water and the waxed (nonpolar) surface, which leads to minimized interactions and a tendency to form spherical shapes.