Final answer:
Slavery in the colonial period established the racial categories in the United States, which were later reinforced by Jim Crow laws and institutional practices, even after the abolition of slavery. The US Census Bureau also contributed to the definition of these categories that continue to affect American society.
Step-by-step explanation:
The historical practice most responsible for creating the racial categories used in the United States is linked to the institution of slavery during the colonial period. The need to distinguish between enslaved individuals and free persons led to the development of legally recognized racial categories that were primarily based on phenotypical characteristics like skin color. Africans became the chosen source of slaves largely due to their physical appearance, which easily fit into the racial criteria established by the legal system at the time. After slavery was abolished, the racial categories were perpetuated and redefined through the Jim Crow laws and other institutional practices, including military segregation and blood quantum rules. The US Census Bureau has also played a role in shaping these categories throughout history. Despite efforts towards equality, these categories continue to influence American society, as seen through practices like racial steering and implicit biases in education.