Answer: The lines from "Mutability" that reflect the idea that life is fleeting are "We are as clouds that veil the midnight moon;
How restlessly they speed, and gleam, and quiver"
Step-by-step explanation:
In these lines, the writer makes a comparison between people and clouds. Shelley uses this metaphor to refer to the human life as something brief and fleeting. Life comes and goes in just one moment. The same happens to the clouds, they appear at night out of the sudden, they veil the moon and they "speed, gleam and quiver" restlessly. Clouds are fleeting just like human life.