Which of the following substances is insoluble in water?

Chalk powder

The correct answer is option 1. Chalk powder.

Chalk powder is primarily composed of calcium carbonate \((CaCO_3)\), which is sparingly soluble in water. However, compared to the other substances listed (sugar, milk, and salt), chalk powder (calcium carbonate) has relatively low solubility in water.

When chalk powder is added to water, only a small amount of it will dissolve, forming a suspension rather than a true solution. Most of the chalk powder particles will remain suspended in the water without fully dissolving. This is because calcium carbonate has limited solubility in water due to its relatively low solubility product.