What is attributed to the formation of rime icing?

Rime icing is primarily formed when supercooled small water droplets come into contact with an object, such as an aircraft. These droplets remain in liquid form even at temperatures below freezing. When such droplets collide with the cold surface of an aircraft, they freeze upon impact, creating rime ice. This type of icing typically occurs in conditions such as low clouds or fog where supercooled water is prevalent.

The other options do not correctly explain the phenomenon. Warm air with high humidity would typically lead to a situation where the water would not necessarily freeze upon impact. Excessive altitude gain can lead to other types of icing or atmospheric phenomena, but it does not directly cause rime icing. Heavy precipitation involves larger water droplets, which are more likely to create different forms of icing rather than rime icing specifically, which relies on the interaction with supercooled droplets.