Answer:
Shelley conveys the idea that humans are constantly moving and changing:
B. by comparing humans to clouds.
Step-by-step explanation:
We are as clouds that veil the midnight moon;
The very first line has the answer we need. Shelley is using a type of comparison here called simile. His purpose is to confer some of the attributes of the clouds to us humans by saying we are like each other. Humans also "speed, and gleam, and quiver," figuratively speaking. We are, like the clouds, in constant motion, ever changing. One day we also shall be gone, just like the clouds once the night closes round.