Final answer:
Cecil Rhodes, the Colonial Secretary at the Cape, is the one who declared the importance of the rock for South Africa's future, which is associated with his imperialist and expansionist ambitions in the region.
Step-by-step explanation:
The individual who was the Colonial Secretary at the Cape and declared, "This is the rock on which the future success of South Africa will be built", is widely attributed to Cecil Rhodes. Rhodes was instrumental in expanding British influence in Africa during the late 19th and early 20th centuries. Known for his imperialist views and the pursuit of British expansion, he was deeply involved in introducing British settlement and control over southern Africa, promoting the 'Cape to Cairo' railway project for this purpose. His statement reflects his belief in colonial expansion as the foundation for South Africa's future.
Rhodes was also a figure associated with systemic racism and had little respect for the indigenous African people. His vision for the British Empire in South Africa, which included the ambition to develop transportation and communication lines like the 'Cape to Cairo' railway, was tied to his goal of strengthening British imperialism and attracting more settlers, as well as his own personal gain. Regardless of Rhodes' impact and motivations, his name is inexorably linked with the colonial history of South Africa and the region that was once named Rhodesia, now known as Zambia and Zimbabwe.