Which one of the following is not a form of precipitation?

Precipitation refers to any product of the condensation of atmospheric water vapor that falls under gravity from clouds. Hail, snowfall, and rainfall are all forms of precipitation. Fog, however, is not precipitation - it is a colloid of water droplets suspended in air near the Earth's surface, formed by condensation without reaching the ground as falling droplets. Fog is a type of cloud touching the ground, not falling from it.