Who kills Curly, prompting Buck to retaliate?

The correct answer is Spitz. In "The Call of the Wild," Curly is a dog who is part of Buck's sled team. Curly's tragic death occurs during an encounter with a group of wild dogs, which are led by Spitz. When Curly tries to playfully engage with another dog, she is attacked, and Spitz, who is the lead dog, plays a critical role in this attack, escalating the violence. This moment is pivotal as it exposes Buck to the brutal realities of survival in the wild and sets off a chain of events that leads Buck to retaliate against Spitz, who embodies the harsh law of the wilderness. Buck's response to Curly's death marks a significant transformation in his character as he begins to understand and embrace the instincts necessary for survival.