What is Buck’s final act of loyalty after Thornton's death?

Buck’s final act of loyalty after Thornton's death is to seek vengeance against his killers. This action illustrates the deep bond that developed between Buck and Thornton throughout the story. Thornton represented a moment of pure love and care for Buck, contrasting with the harshness he experienced earlier in his life. When Thornton is killed, Buck’s instinctual response is driven by loyalty and the need to avenge his beloved owner. This marks a significant turning point for Buck, showing how his character has evolved from a domesticated dog to a creature embracing his primal instincts.

The other options highlight different behaviors Buck might exhibit, but they do not capture the specific act of loyalty that stems from his relationship with Thornton. Stealing food from other camps, running away into the wilderness, or howling for help reflect survival instincts and reactions to his circumstances rather than the direct loyalty to Thornton that motivates his vengeful pursuit. Buck’s actions are not merely about survival but rather an emotional response to the loss of someone he loved, indicating the depth of his loyalty.