When you have a two-layer mixture of water and tert-butyl methyl ether, what physical property of these liquids assures you that the ether is the top layer?

The density.

Explanation:

The density is an intensive property of fluids, defined as mass divided by volume.

This property is directly related to the intermolecular forces that are present in the substance: the stronger the intermolecular forces, the higher the density will be, and viceversa. Therefore, organic fluids usually have lower densities than water, because the intermolecular forces in organic fluids are not as strong as in the intermolecular forcer in water (water has very stable hydrogen bonds, whereas organic fluids have van der Waals, weaker than hydrogen bonds).

The liquid that has the lower density will be the the liquid that is on the top layer. In this case, the organic compound tert-butyl methyl ether will be on top of water.