Final answer:
Buck undermines Spitz's position as pack leader through direct confrontation and challenging the established hierarchy by picking fights with him.
Step-by-step explanation:
The question pertains to Jack London's The Call of the Wild and focuses on how Buck begins to challenge Spitz's authority as the pack leader. In the narrative, Buck starts to undermine Spitz's position as the leader of the dog pack not by protecting other dogs from Spitz, becoming friends, or deciding on co-leadership, but through direct confrontation. Buck intentionally defies Spitz's authority and picks fights with him, asserting his dominance and challenging the established hierarchy within the pack.