Explanation:

- Option 1: Copper sulphate

- When dissolved in water, it forms a true solution.

- In a true solution, the particles are too small to scatter light.

- This means it does not scatter a beam of light.

- Option 2: Chalk powder

- Chalk powder forms a suspension in water.

- In suspensions, the particles are large enough to scatter light.

- Option 3: Milk

- Milk forms a colloidal solution.

- Colloids have particles that can scatter light.

- Option 4: Ink

- Ink forms a type of colloid when mixed with water.

- It contains particles that can scatter light.