Which material is used as a liquid pigment in writing instruments?

Ink is the correct answer because it is specifically formulated as a liquid pigment for use in writing instruments. It consists of colorants, often dyes or pigments, suspended in a liquid medium that allows it to flow smoothly through pens, markers, quills, and other writing tools. The formulation of ink enables it to adhere to paper and create a permanent or semi-permanent mark, depending on the type of ink used.

In contrast, paint, while it also contains pigments, is typically used for larger applications like artwork and not specifically as a liquid for writing. Graphite is a solid form used in pencils and is not liquid, functioning by leaving a mark through physical abrasion. Charcoal, another solid medium, is used in drawing and also does not have the liquid consistency needed for writing instruments. These distinctions clarify why ink is the appropriate answer in this context.