(a) The sentence that best states a theme of the passage is:
C. Adventure can occur unexpectedly.
(b) The detail from the passage that best supports the answer to Part A is:
A. ". . . and was just in time to see it pop down a large rabbit-hole under the hedge." (paragraph 3)
This detail highlights the unexpected nature of Alice's adventure as she suddenly decides to follow the rabbit down the hole, leading her into a new and adventurous experience.