Question

which excerpt from lord Byron's "she walks in beauty" most directly describes its subject as a part of all nature?

Answer 1

The excerpt from Lord Byron's "She Walks in Beauty" that most directly describes its subject as a part of all nature is the following one:
“…like the night/Of cloudless climes, and starry skies…”
This is the only excerpt that even remotely mentions nature, let alone compare the woman to the night, and cloudless climes, and starry skies. You can see that she is inseparable from nature, because nature is beauty and love, the same way she is.

Answer 2

A. "...like the night/Of cloudless climes, and starry skies..."