A short answer would be "All of the above." At first I thought this was a case of "flammable" versus "inflammable" which oddly both mean the same thing: burnable. According to the dictionary, however(for this is really a word thing, not a science thing), homogeneous means "of uniform structure throughout" while heterogeneous means "a mixture consisting of dissimilar or diverse ingredients". Both of these describe milk perfectly, since everyday homogenized milk IS a uniform mixture of dissimilar ingredients (fats, water, solids, etc.) This might not be true of raw milk which tends to separate into the aqueous and fatty parts, but for the milk most of us know, it's accurate.