Final answer:
The last line from the stanza in "Mutability" implies that change is the only constant and inevitable aspect of life, making option b the correct answer.
Step-by-step explanation:
The idea conveyed in the last line of the stanza from "Mutability" is that change is the only constant in life. The phrase "Nought may endure but Mutability" suggests that everything is subject to change and nothing remains the same forever. Therefore, the best answer to the question is:
b. Change is the only thing that is eternal.