Explanation:

The scattering of light in a mixture depends on the particle size of the substance added to water. Substances that dissolve completely and form true solutions do not scatter light, whereas those forming colloids or suspensions do.

- Option 1: Copper sulphate - When added to water, copper sulphate dissolves completely, forming a true solution where particles are too small to scatter light. Correct Answer

- Option 2: Chalk powder - Forms a suspension with larger particles, causing light scattering.

- Option 3: Milk - Contains colloidal particles (such as fat globules) that scatter light.

- Option 4: Ink - Typically forms a colloid where the particles are large enough to scatter light.