Which group in nineteenth-century America would likely be the most receptive audience for "What to the Slave is the Fourth of July?"

Abolitionists

Explanation:

The speech "What to the Slave is the Fourth of July" by a former slave, Frederick Douglas, focused on showing the Independence Day and its principles such as freedom or liberty in the U.S. did not protect slaves, and therefore the ideals held by many only applied to part of the population. This tried to show the audience the importance of ending slavery and providing slaves the freedoms white people had.

In this context, this speech would be supported by abolitionist (people that wanted to end slavery) and they would be more receptive to it because they supported the idea of providing freedom and protections to slaves and ending enslavement, which is the focus of the speech.