How does Buck ultimately become the lead dog?

Buck ultimately becomes the lead dog after he defeats Spitz in a fight. This confrontation is pivotal in the story as it embodies Buck's evolution from a domesticated pet to a leader in the wild. Spitz, who is the current lead dog, represents the old order and an aggressive competitor in the harsh environment of the Yukon. The fight is not just a physical battle; it symbolizes Buck’s growing instincts and strength, honed by his experiences in the wild.

By overcoming Spitz, Buck demonstrates his dominance and capability, which earns him respect from the rest of the team and establishes him as the new leader. This transition marks a significant turning point for Buck as he fully embraces his primal instincts and leadership role, which are central themes in Jack London’s exploration of survival and the struggle for dominance in nature. The other options do not reflect the circumstances leading to Buck's promotion to lead dog, as they do not relate to his direct challenge and conquest of Spitz.