Final answer:
The text that conveys the idea that national parks belong to all Americans is the passage that mentions the joy and pride of the American public in their national parks and their development as a valued economic asset.
Step-by-step explanation:
The detail in the text that helps unfold the idea that the United States' national parks belong to all Americans is found in excerpt 14. This passage explains the sense of pride and joy that the American public feels in realizing the treasure that the national parks represent, and the confidence of the Government in the parks' rapid development as a public and economic asset. It captures the sentiment that the discovery and appreciation of these natural wonders foster a sense of collective ownership and national identity.