Frost on an airframe typically forms under which conditions?

Frost on an airframe typically forms when water vapor undergoes deposition on a cool surface, which is a process that occurs when water vapor in the air comes in contact with a surface that is below the frost point temperature. This causes the vapor to transition directly into solid ice, resulting in the frost formation. This phenomenon is particularly common under humid conditions when the temperature of the surface has dropped sufficiently.

While below-freezing temperatures can lead to frost and might suggest some connection to its formation, it is specifically the deposition of water vapor that is the primary mechanism for frost to form. Options mentioning freezing of water vapor during flight or the presence of super-cooled water droplets focus on different phenomena that are not the direct cause of frost, as they involve either different physical processes or specific conditions not necessarily tied to frost formation on an airframe.