Final answer:
The ocean waves threatening to break New York City's levees most clearly demonstrate nature's power as they can potentially overcome human-made protective structures.
Step-by-step explanation:
The detail from "The City Without Us" that most clearly shows that nature is more powerful than it appears is b. The ocean waves that threaten to break the levees around New York City. This detail illustrates the immense force of nature that can potentially overcome human-engineered structures meant to protect against natural elements. The other details provided, such as the size of the trees, demonstrate nature's majesty and presence, but they do not convey the same level of power over human constructs as the description of the ocean's force does.