Answer:
Gifford Pinchot's significant contribution to environmentalism is:
a.) Served as the first chief of the U.S. Forest Service
Step-by-step explanation:
Gifford Pinchot was the first Chief of the U.S. Forest Service, and he made substantial contributions to the conservation and sustainable management of forests and natural resources in the United States. While other options (b, c, and d) are essential aspects of environmentalism, Pinchot's role as the first Chief of the U.S. Forest Service is his most notable contribution to the field.