The problem is telling us that the substance is not chemically combined, so this means that it doesn't represent a compound because a compound is a substance that contains two or more elements that are chemically combined.
A heterogeneous mixture isn't uniform in a composition, so this isn't the answer.
An element has the same atoms in the same place so this isn't the answer either.
In this case, the answer is a homogeneous mixture, because the composition is uniform throughout the mixture.
