Final answer:
The primary medium of Wenda Gu's United Nations—China Monument is human hair. So, the correct option is D.
Step-by-step explanation:
Gu used collected hair to create panels representing different nations, promoting a message of global unity.
Wenda Gu is known for his large-scale installations that often utilize unconventional materials. In this work, he collected human hair from various barber shops around the world and constructed a series of panels that represent different nations, symbolizing global unity and the interconnected nature of humankind. These hair panels are hung in a way that they form a monumental structure, challenging traditional notions of sculpture and materials used in art.
Therefore, D. Human hair is the correct option.