The correct answer is C. The white population of the United States
The audience of a text or speech is the group of people the author is addressing to during a speech or text as the ideas or topic the author is addressing concerns to this specific group of people or is related to it. Determining to whom is the author addressing is not always easy as it is not usually stated explicitly but suggested through the context of the text or speech and the language used in it. In the case of the excerpt from “The Hypocrisy of American Slavery,” the author criticizes the celebration of Fourth of July and other common celebrations in the U.S. as he believes these celebrations just show the injustice and cruelty slavers had suffered and not the freedom the people claim. Additionally, during all the text the author emphasizes the barbarity in the U.S. and even states this country abuses do not compare to the abuses in any other country.
This implies the author is criticizing and addressing to the oppressor of slavers and considering he is part of the abolitionist movement and the context of the text it can be concluded he is addressing to the white population in the U.S. as it is this part of the population the one that is responsible for the abuses and all the elements the author criticizes in the text.