Final answer:
Simon Bolivar is the historical figure who declared the severance of South America's ties to Spain in an 1815 letter. He played a central role in the liberation of South America from Spanish rule and advocated for a united, republican continent.
Step-by-step explanation:
The individual who declared in an 1815 letter that the tie binding South America to Spain had been severed is C) Simon Bolivar. Bolívar was integral to the South American independence movement, leading the charge against Spanish control. In his 'Letter from Jamaica,' written after the Spanish attempt to reassert authority in South America following Napoleon Bonaparte's defeat, Bolívar outlined his vision for a unified Spanish America free from colonial rule. He was a key figure who led the fight for independence and helped in liberating territories that comprised Gran Colombia.
Simon Bolivar's role was crucial in the geopolitical sphere of South American nations, highlighting a broad continent-wide outlook for alliances among the newly independent states. Unlike others who admired the British constitutional monarchy, Bolívar favored a republican system, seeing it as a guarantee for stability and advocating for the independence of South American territories.