The correct option is A colloid
A colloid is a heterogeneous mixture. It has suspended particles in the medium that can be seen under a microscope.
Milk appears to be a homogeneous mixture but it is a heterogeneous mixture. It consists of small globules of fat, proteins, and other minerals which do not settle down when left undisturbed. They are dispersed unevenly throughout the mixture. This is why milk is a colloid.