Ellen Churchill Semple, an influential American geographer, is credited with defining Human Geography as "the study of changing relationships between the unresting man and the unstable earth." This definition encapsulates the essence of her environmental determinist perspective on the field of human geography.

Semple's view of human geography emphasized the intricate and dynamic interactions between human societies and their physical environment. She believed that environmental factors, such as climate, topography, and natural resources, played a significant role in shaping human culture, society, and behaviors.

 Her definition suggests that humans are in a continuous and evolving relationship with the ever-changing and often challenging natural world.

THEREFORE THE CORRECT OPTION IS [3].