Final answer:
Cirrus clouds are the wispy clouds found at the highest elevations in Earth's atmosphere, made of ice crystals and differing from lower-elevation cumulus clouds.
Step-by-step explanation:
The wispy clouds that exist at the HIGHEST elevation are known as cirrus clouds. These clouds are characterized by their thin, featherlike appearance and are made of ice crystals.
They typically form when water vapor freezes at high altitudes. In contrast, cumulus clouds form due to the rising of water vapor because of convection, and they indicate lower elevations than cirrus clouds.
The information provided suggests that cirrus clouds can even be found on other planets, such as Neptune, where they are made of methane ice crystals. Thus, the answer to the student's question is c) Cirrus.