Final answer:
Graphite is a form of carbon, which is an element on the periodic table, making it the correct answer.
Step-by-step explanation:
The student's question asks which of the given substances is an element. The correct answer is b. graphite. Graphite is a form of carbon (C), which is listed on the periodic table as an element. Examples of elements include uranium, gold, and carbon, among others. Substances like brass and steel are alloys, which means they are a mixture of elements rather than a pure element. Plastic is a polymer made of long chains of repeating units, not an element. Wood is a composite of cellulose fibers (which are organic polymers) and other organic compounds, hence not an element either.