Final answer:
Greenpeace is identified as the organization most likely to have supported the environmental movement during the 1960s to 1980s, as it represents environmental interests and advocates for environmental protection.
Step-by-step explanation:
According to Parenti, the organization that was most likely to support the growing environmental movement in the 1960s through the 1980s was Greenpeace, representing environmental interests. This organization, along with others, lobbied for environmental protection measures like the ban on seal hunting, highlighting the sustainability and cruelty concerns associated with such practices. Interest groups are typically categorized as either economic or public interest groups, with the former focusing on industry protections, wages, and job creation, while the latter, like Greenpeace, advocate for issues such as environmental protection that are considered to serve the public good.