What motivates the character of Manuel to sell Buck?

The motivation behind Manuel selling Buck is primarily driven by his need to pay off debts. Throughout the narrative, it is evident that Manuel is in a precarious financial situation. Selling Buck, who is a valuable and strong dog, represents a quick way for him to alleviate his debt burdens. This choice reflects Manuel's desperation and lack of loyalty to Buck, illustrating the harsh realities of life during the time period depicted in the story.

The other motivations do not align as closely with Manuel's circumstances. For instance, the desire for companionship contrasts with Manuel's decision to discard Buck for financial gain. Additionally, while some individuals might enjoy dog fighting, that is not depicted as a motivation for Manuel in this scenario. Finally, proving his worth as an owner does not resonate with his actions; instead, he demonstrates a disregard for Buck’s well-being in favor of his financial needs. Thus, the correct answer reflects the driving force behind Manuel's choices in the context of the story.