Concept:The decline of Boer power in South Africa came from the political and military supremacy of the British.
Explanation:Mfecane was a period of chaos among indigenous Bantu communities, not the Boers, so it does not explain the Boer decline.
The Great Trek was a Boer movement into the interior to escape British rule, which actually increased their independence temporarily.
Dutch domination ended before the Boers established their republics and bears no direct relevance to the later loss of Boer influence.
The British expanded control over Southern Africa, defeated the Boer republics in the Anglo-Boer Wars, and eventually united the region under British authority.
Thus, it was British domination that directly reduced the political influence of the Boers in South Africa.
Answer:British Domination
(Option C).