The most effective course of action to decrease the impact of urbanization on the ground squirrel population would be:
D. creating protected areas where human activity is limited.
Creating protected areas can provide a safe habitat for the ground squirrels, reducing their exposure to urban development and human activities that can lead to habitat loss and fragmentation. This approach can help sustain their population and promote biodiversity in the urbanizing area. Options A, B, and C may have negative consequences or fail to address the core issue of habitat loss.