Final answer:
Option (a), Theodore Roosevelt's philosophy on environmental conservation was the utilitarian conservation of natural resources for future generations, which included creating reserves, sanctuaries, and parks, with some allowance for public development.
Step-by-step explanation:
President Theodore Roosevelt's personal philosophy when it came to environmental conservation was primarily based on the concept of utilitarian conservation, a notion that aligned closely with the views of Gifford Pinchot, Chief Forester under his administration. Roosevelt advocated for the preservation of natural resources for future generations (a), as evidenced by his actions to set aside millions of acres for forest reserves, establish wildlife sanctuaries, and initiate several national parks.
However, unlike John Muir's more purist preservationist approach, Roosevelt did allow for some development of wilderness areas for the common good, demonstrating a balance between conservation and public utility.