Final answer:
The graphite of a pencil writes on paper due to the frictional force, which is an external force that transfers particles of graphite to the paper's surface.
Step-by-step explanation:
The force that causes the graphite of a pencil to write on paper is c) Frictional force. When you move a pencil across a surface, the friction between the graphite and the paper causes tiny particles of graphite to leave the pencil and stick to the paper, which results in writing or drawing visible marks. Friction is an external force that acts at the interface between two surfaces in contact, as characterized in Newton's First Law of Motion related to inertia.