* Physical Changes: Involve changes in the form or appearance of a substance, but not its chemical composition. The substance itself remains the same.

Let's break down the options:

* A. An iron bar rusting: Rusting is a chemical change. Iron reacts with oxygen and water to form iron oxide (rust), a completely different substance.

* B. Oil floating on water: This is a physical change. Oil and water are simply two different substances with different densities, causing one to float on the other.

* C. Copper skillet tarnishing: Tarnishing is a chemical change. Copper reacts with oxygen and other elements to form a layer of copper oxide, which is a different substance.

* D. The sublimation of dry ice: Sublimation is a physical change. Dry ice (solid carbon dioxide) changes directly into gaseous carbon dioxide. The chemical composition remains the same (CO2), only the state of matter changes.

In summary, only sublimation of dry ice is a physical change because it alters the state of matter without changing the chemical composition of the substance.