What action does Buck take to save John Thornton?

Buck's decision to jump into the river is a pivotal moment in the story as it showcases his deep bond with John Thornton and his bravery. At this point in the narrative, Buck has fully embraced his instincts and has developed a strong loyalty to Thornton, who has treated him with kindness and respect. When Thornton falls into the treacherous waters, Buck's instinctual response is not just to bark or signal for help; instead, he leaps into action, literally diving into the river to rescue him.

This act illustrates Buck's transformation from a domesticated pet to a wild creature who is in tune with his instincts and willing to take significant risks for his family, portraying themes of loyalty, courage, and the bond between man and dog. The choice captures the essence of Buck's character development throughout the novel, emphasizing his capacity for love and sacrifice.