Night came on, and a full moon rose high over the trees into the sky, lighting the land till it lay bathed in ghostly day. And with the coming of the night, brooding and mourning by the pool, Buck became alive to a stirring of the new life in the forest other than that which the Yeehats had made. He stood up, listening and scenting. From far away drifted a faint, sharp yelp, followed by a chorus of similar sharp yelps. As the moments passed the yelps grew closer and louder. Again Buck knew them as things heard in that other world which persisted in his memory. He walked to the centre of the open space and listened. It was the call, the many-noted call, sounding more luringly and compellingly than ever before. And as never before, he was ready to obey. John Thornton was dead. The last tie was broken. Man and the claims of man no longer bound him. —The Call of the Wild,
Jack London

What change in Buck’s character occurs at this moment of the story and leads to the resolution?

Buck leaves the world of humans and becomes connected to the wild.
Buck decides that he wants to go back to his previous life.
Buck no longer cares about John Thornton.
Buck realizes that he can never survive in the wild without humans.