Why is milk regarded as a mixture and not a pure substance?

Pure substances are either elements or compounds while the mixture is a resultant product of any two or more substances.

Milk is a mixture of fat and protein dissolved in water.

A colloid is a type of solution in which the size of particles is not visible to the naked eye but they can scatter the beam of light. A colloid solution appears to be homogenous but it is actually a heterogeneous mixture.

Milk is actually a type of colloid solution as it has fat and protein dissolved in water and these solid solute particles are not visible but they can scatter the light.

Hence, the correct option is (A).