What are both Henry David Thoreau and George Perkins Marsh called? 

A)  Native Americans 
B)  industrialists 
C)  land barons 
D)  conservationists

Henry David Thoreau

A 19th-century American essayist, poet, and philosopher, best known for his book "Walden," a reflection upon simple living in natural surroundings.

George Perkins Marsh

A 19th-century American diplomat and environmentalist considered one of the precursors to the modern environmental movement.

Conservationists

Individuals or groups dedicated to the preservation and protection of the environment and wildlife.