Out of the given options, zinc oxide, chalk, and white lead are all pigments. Pigments are substances that are used to add color to materials such as paints, inks, and dyes.

Zinc oxide is a white pigment commonly used in paints, ceramics, and cosmetics. It provides a bright, opaque white color.

Chalk, also known as calcium carbonate, can be used as a pigment in paints, pastels, and colored pencils. It is a soft, white substance that is often used to create light shades of color.

White lead, also known as lead carbonate, was historically used as a white pigment in oil-based paints. However, due to concerns about toxicity, its use has been reduced and regulated in recent years.

On the other hand, silica is not a pigment. It is a naturally occurring mineral that is commonly used as a filler or extender in paints and other materials. It improves the durability and performance of paints but does not provide color. Silica is usually white or transparent in its pure form.

Therefore, option 4, silica, is the correct answer as it is not a pigment.