Final answer:
The most successful American filibustering endeavor occurred in Nicaragua, primarily through William Walker's actions.
Step-by-step explanation:
The nation that experienced the most successful American filibustering endeavor was b. Nicaragua. William Walker's exploits in Nicaragua are perhaps the most well-known and significant example of American filibustering in the 19th century. In comparison, while Narciso López did garner American support in his attempts to annex Cuba, his repeated efforts ultimately failed, and he was executed by Spanish authorities without managing to incite a massive uprising among the Cuban people. Neither Cuba nor Belize endured a successful American filibustering attempt. As for Mexico, there were several filibustering incidents such as the Long Expedition but none resulted in long-term success.
In the context of discussing the consequences and moral aspects of such events, it would be interesting to consider how people in Cuba or Nicaragua might have viewed these actions. Given the aim to spread slavery and the aggressive nature of these expeditions, support from the local population would not be guaranteed, often leading to conflict and uncertainty within the region targeted by filibusters.