Final answer:
Greenpeace is classified as a public-interest group because it focuses on broad issues that affect the general public such as environmental protection, rather than representing specific economic interests.
Step-by-step explanation:
The category of interest group that Greenpeace falls into is that of a public-interest group. This classification is due to Greenpeace's primary mission, which revolves around promoting environmental protection and advocacy that transcends particular economic interests for the greater benefit of the public and the environment. These groups, which also include noneconomic interest spheres such as civil rights and ideological interests, focus on broad issues that affect the general public as opposed to specific economic sectors. They engage in activities ranging from research and advocacy to mobilization of public opinion and policy change efforts.
Moreover, Greenpeace, like other public-interest groups, tends to participate in coalitions such as the Save Our Environment Action Center, which enables various environmental organizations to collaborate, pool resources, and extend their reach and impact. As a historically significant and globally recognized organization, Greenpeace has been active in various campaigns against practices they deem harmful to the environment, such as the ban on seal hunting in the 1970s, and continues to promote greener alternatives in various industries today.