Both individual citizens and interest groups often write letters to their representatives to express concerns about policy issues. Individual citizens may write based on personal opinions or experiences, while interest groups advocate for specific causes. Therefore, the correct answer to the question is option D: interest groups and individual citizens.
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Individual citizens and interest groups are the primary writers of letters to representatives about policy issues, allowing them to express concerns and influence legislation. The media and political parties typically do not directly engage representatives in this way. Thus, the most accurate answer choice involves both citizens and interest groups. ;