Concept:A pigment is a coloured substance that is nearly insoluble in water and used to impart colour to materials.
Explanation:To determine which is not a pigment, check each option for its primary function and colouring property.
Zinc oxide (
ZnO) is a white pigment known as zinc white, used in paints.
Chalk (calcium carbonate,
CaCO3​) is a soft white rock; its purified form (whiting) is used as a pigment and filler.
White lead (basic lead carbonate) is a heavy, poisonous white pigment historically used in paints.
Silica (silicon dioxide,
SiO2​) is a colourless mineral that forms quartz. It is not used as a colouring pigment; instead, it is employed in glass, ceramics, abrasives, and as a filler in grounds or primers. Silica lacks the characteristic of being a coloured substance for direct pigmentation.
Hence, among the four, only silica is not a pigment.
Answer:D. Silica