Physics - Elasticity Question with Solution | TestHub

The change in length due to thermal expansion is given by $\Delta L=L_0\alpha \Delta T$. Since both copper bars have the same initial length ($L_0$), are made of the same material (copper, so $\alpha$ is the same), and are heated to the same temperature ($\Delta T$ is the same), the change in length ($\Delta L$) will be the same for both, regardless of their diameter.

The final answer is $\boxed{C}$.