The sample from the bottom has more of the substance than the sample from the top. what is this mixture?

This mixture would be a SUSPENSION.  

Suspensions are mixtures in which the solute remains insoluble in the solvent and as such the solute and the solvent remain miscible.  In many cases, and in this case, the solute is denser than the solvent, and as such, it sinks to the bottom of the container.

An example of this would be a mixture of sand an water where it can be seen where the sand sinks to the base of the container with water.

They are all solids.

The answer is d.potassium