The correct answer is (a) graphite.
Graphite is a form of carbon that is soft and has a low friction coefficient. This means that it does not stick to other surfaces very well and can easily slide over them. This makes it ideal for use in pencils, as it allows the lead to glide smoothly across the paper.
Charcoal, lamp black, and coal are all forms of carbon that are harder than graphite. They also have higher friction coefficients, which means that they are more likely to stick to other surfaces. This makes them less suitable for use in pencils, as they would cause the lead to drag and scratch the paper.
In addition, graphite is a good conductor of electricity, which allows it to carry the electrical current that is necessary to power the eraser on the end of a pencil. Charcoal, lamp black, and coal are not good conductors of electricity, so they would not be able to power the eraser.
Therefore, the correct answer is (a) graphite.