Which substance is used for making pencil lead? 

Graphite 

The correct answer is option 3. Graphite.

Graphite is a form of carbon that is known for its softness and ability to leave marks on surfaces. It is composed of layers of carbon atoms arranged in a hexagonal lattice structure. These layers are held together by weak van der Waals forces, allowing them to easily slide over each other.

When graphite is used in pencil lead, the layers of carbon atoms shear off onto the paper as the pencil is applied, leaving behind a mark. The softness and smoothness of graphite make it ideal for writing and drawing purposes. Additionally, graphite is relatively inert and does not react with paper or other materials, making it a stable and reliable choice for use in pencils.

In contrast, materials like sulfur, aluminum, and silicon do not possess the same characteristics as graphite and are not suitable for making pencil lead.