Which of the following best defines "graphite"?

The best definition of "graphite" is that it is a soft grey piece of carbon used in making pencil lead. Graphite is a naturally occurring form of carbon, known for its excellent conductivity of heat and electricity, as well as its lubricating properties. This unique structure allows it to be easily shaped into a fine point, which is ideal for pencil lead. The soft nature of graphite gives it its characteristic ability to leave a mark on paper when pressure is applied, making it a staple in writing and drawing instruments.

While the other options describe materials or tools related to drawing or writing, they don't accurately capture the essence of graphite itself. Graphite is specifically recognized for its role in producing pencil lead, which is why the choice identifying it as a piece of carbon in this context is the most precise.