We live.—Knowing that today’s sun may give way to tomorrow’s rain.
The passage talks about how things are always changing. The only thing that is the same is the fact that things change. We see this in the last line of the passage when it says "Nought may endure but Mutability." Mutability is the tendency to change. It's saying that nothing endures (or lasts) like the tendency to change. The correct answer should continue this idea. Option B talks about how we know things will change from one day to the next with one day being sunny and the next rainy.
We live.—Knowing that today’s sun may give way to tomorrow’s rain.
Explanation:
In these stanza Bruno describes how the only thing that endures is the mutability of the human being, he describes how emotions are ever changing and so does the human being, we live for change, watching, promoting and expecting how everything change with every action.