High-level clouds usually occur above 20,000 ft. which is the best description of the clouds’ appearance?

Thin and composed of ice crystals.

Explanation:

According to a different source, these are the options that come with this question:

The high-level clouds that occur above 20,000 ft. are known as cirrus clouds. Cirrus clouds are the most common of all the high clouds, They are long, thin, wispy streamers that are composed of ice. Moreover, these clouds are usually very white and they tend to appear when the weather is fair.