Final answer:
The 500,000 free blacks living in the antebellum period were largely due to manumission and gradual emancipation laws that occurred prior to the Emancipation Proclamation, with a significant number being in states like Louisiana before the Louisiana Purchase and others coming from regions outside the U.S. such as Cuba and Haiti.
Step-by-step explanation:
The reason for the existence of 500,000 free blacks during the antebellum period is c) Manumission and gradual emancipation laws.
The antebellum period saw a significant number of free Black individuals due to various factors, including manumission -- the formal granting of freedom to slaves -- which gained momentum after the American Revolution as ideals of equality spurred slave owners to free their slaves. Additionally, the agricultural shift in the Upper South from labor-intensive crops to wheat, which required fewer slaves, led to an increase in manumissions. Other factors contributing to the free Black population were the historical free Black residents of Louisiana before the Louisiana Purchase and immigrants from places like Cuba and Haiti.
It is noteworthy that gradual emancipation laws in states like Pennsylvania, Vermont, Massachusetts, and New Hampshire also facilitated the rise of the free Black population, well before the Emancipation Proclamation in 1863, which only freed slaves in states that were still in rebellion against the Union.