Answer:
c) oil and vinegar
Step-by-step explanation:
A heterogeneous mixture is a mixture that does not have a "uniform composition" (meaning that the entire mixture is not thoroughly mixed to be the exact same. In different parts of the mixture, there is a non-consistent / differing composition.)
Oil and vinegar, as you may know, do not thoroughly mix when put together--different parts of the mixture are made of different compositions. Some parts are made of oil, and some are a concentration of vinegar, but because they are not able to be mixed entirely, the mixture is considered a heterogeneous mixture. If you were to take different samples of different parts of the liquid, you would have a clear separation of oil and vinegar.
Milk is thoroughly milk--if you take out a tablespoon, it will be milk entirely--and it will have the same composition as all of the other milk in the bottle.
Air is complicated--and it can be described as either, but the question was likely intending the answer of oil and vinegar.
Vinegar and water do thoroughly mix. Vinegar absorbs water on a molecular level, so the mixture is thoroughly mixed when put together.
(Hope this helps!)