Final answer:
Buck's character grows as he embraces his true nature, akin to that of a wild wolf, and accepts his destiny, as indicated by option (d).
Step-by-step explanation:
The character growth of Buck in the excerpt can be seen as he comes to terms with his own nature and place in the world. This realization is best captured by option (d) He decides that he is as wild as a wolf and accepts his destiny. The excerpt portrays a moment of profound self-awareness and acceptance for Buck, as he acknowledges the immutable truth of his existence and his pride in overcoming the forces of man and nature alike.