Which of the following materials is considered a nonmetal?

Carbon is classified as a nonmetal due to its position on the periodic table and its chemical properties. Nonmetals, such as carbon, typically exhibit poor electrical conductivity, a lack of luster, and a tendency to gain electrons during chemical reactions. Carbon is essential for organic chemistry and is a fundamental building block of life, forming a vast array of compounds through covalent bonding.

In contrast, gold and copper are both metals, characterized by their excellent conductivity and malleability. Sodium, while it's a metal as well, is in the alkali group, known for its reactivity with water. Thus, carbon's distinct properties and its place on the periodic table define it as a nonmetal.