Final answer:
Interest groups in the United States have increased in number and influence throughout the 21st century, reflecting a vast array of interests across society and employing sophisticated lobbying efforts to shape public policies.
Step-by-step explanation:
When considering the conclusion that can be drawn regarding the number of interest groups in the United States in the 21st century, the most accurate statement is that interest groups have grown in number and influence. A substantial increase in both the number and type of groups has been observed, with diverse interests being represented at various levels of the government. The passage of the Lobbying Disclosure Act in 1995 has improved the understanding of these numbers in Washington, DC, and subsequent counts have made it evident that interest groups are plentiful and varied.
Interest groups have become more sophisticated, hiring lobbying firms to represent their interests, and reflect the increasing specialization of industry and fragmentation of interests. This leads to a scenario where a multitude of interests, ranging from corporate to public interest groups, like the U.S. PIRG, are all actively seeking to influence government policies.
Moreover, the potential for all active and legitimate groups to make themselves heard is subject to debate. While there is a growth in numbers, the influence of interest groups can lead to the prominence of certain interests, which might not necessarily reflect a democratic majority.