Option 4, which suggests comparing social media usage in South Africa with that in the United States, is incorrect because it would divert Nkosi's research from its main focus on youth activism within a specific cultural and political context. By introducing a comparative analysis with the United States, the scope of his research expands considerably, which may lead to a dilution of the study's key objectives.
Such a comparison would involve examining a wide array of variables, including cultural, social, and political differences between the two countries, which might not be directly relevant to understanding the specific role social media plays in youth activism in South Africa. Instead of providing a focused investigation into how social media facilitates activism among South African youth, option 4 would lead Nkosi to engage in a broader discussion that may not yield insights pertinent to his original research question. Hence, focusing specifically on "the role of social media in facilitating youth activism in South Africa" (option 3) is much more aligned with his objectives.