Final answer:
The true statement about the Great Wall is that it proved China’s resources could be mobilized. It signifies organizational strength of China, not its defense success, the downfall of the Qin dynasty, or the cause of Wudi’s expansionism.
Step-by-step explanation:
Among the statements provided about the Great Wall of China, the one that is true is b. It proved China’s resources could be mobilized. The Great Wall is a testament to China's ability to mobilize and utilize its vast resources for construction projects of monumental scale. The wall stretches over vast distances and is built from a variety of materials including stone, brick, and rammed earth. It involved the labor of hundreds of thousands of workers and showcased the organizational capacity of the Chinese state.
The Great Wall, however, did not always effectively keep invaders out of China, as it did not prevent the Mongols or later the Manchus from invading. Similarly, the wall itself did not cause the downfall of the Qin dynasty; rather, it was the heavy taxation and labor demands associated with such grand projects that contributed to internal strife and rebellion. Furthermore, Wudi's policy of expansionism came much later in the Han dynasty and was not a direct result of the wall but rather a strategic military decision to push Chinese boundaries outward.