An interest group is an organization formed to advocate for specific issues, causes, or interests. In the context of the question, the correct example of an interest group is:
C. The National Association for the Advancement of Colored People
The NAACP is a well-known interest group that focuses on civil rights and racial justice issues, advocating for the advancement and equality of African Americans and other marginalized groups in society.
While NATO, The Republican Party, and The House of Representatives are all important entities, they do not fall under the category of interest groups. NATO is an international alliance, The Republican Party is a political party, and The House of Representatives is a legislative body within the U.S. government.