Milk is a heterogeneous mixture.
Explanation:
Heterogeneous mixtures are non-uniform solutions that can be separated into two (or more) individual substances by physical means (homogeneous mixtures cannot be).
A glass of ice water is a heterogeneous mixture because we can easily separate the ice from the liquid water by filtration.
With milk, you have white particulates that give a non-uniform solution, so it is heterogeneous.