Which process does not involve a change in state: fusion, sublimation, condensation, or thermal expansion?

Thermal expansion is the correct choice because it refers to the increase in the size of an object or substance when it is heated, without any change in its physical state. Unlike fusion, sublimation, and condensation, which involve transitions between solid, liquid, and gas states (for example, fusion is the process of a solid becoming a liquid, sublimation is when a solid turns directly into a gas, and condensation is the process of a gas changing to a liquid), thermal expansion only involves a change in the dimensions of the material due to temperature increases. This means that the substance retains its original state—such as solid, liquid, or gas—while simply expanding. Understanding these concepts is crucial in fields like physics and engineering, where the behavior of materials under various temperatures is fundamental.