Final answer:
Frederick Douglass understood the main reason for white power over blacks during his youth as institutionalized racism, which was supported by a deeply ingrained belief in white supremacy in the antebellum South.
Step-by-step explanation:
When Frederick Douglass was young, he understood the main reason whites held power over blacks as C) Institutionalized Racism. This belief was rooted in the social order of the antebellum South, which was premised on a racial ideology of white supremacy. Despite the varying economic statuses among white southerners, whether they were slaveholders or not, they were united by a shared set of values centered on white superiority.
The social structure was such that white supremacy was used to justify the enslavement and subsequent control over blacks. This ideology persisted even after the abolition of slavery, as evidenced by the implementation of black codes and racial terror designed to maintain dominance over the free black population. The idea of racial superiority was so ingrained that it was passed down through generations, resulting in the long-lasting effects of racism well into modern times.