Thermal Expansion Of Solids

As described in NCERT Class 11, Chapter 11 (Thermal Properties of Matter), most solids expand on heating, and the change in length is given by $\Delta L=L_0\alpha \Delta T$, where $\alpha$ is the coefficient of linear expansion and $\Delta T$ the temperature rise. Heating increases the average separation between atoms, lengthening the rod. The temperature change is $\Delta T=120-20=100{}^{\circ }\text{C}$. Substituting $L_0=1\text{m}$, $\alpha =2\times 10^{-5}{}^{\circ }{\text{C}}^{-1}$ and $\Delta T=100$ gives $\Delta L=1\times 2\times 10^{-5}\times 100=2\times 10^{-3}\text{m}=2\text{mm}$. The option 1 mm uses a temperature rise of only 50°C. The option 4 mm doubles $\alpha$ incorrectly. The option 0.2 mm misplaces a power of ten. As a plausibility check, a millimetre-scale expansion over a metre rod for a 100°C rise is exactly the small but measurable effect that requires expansion gaps in railway tracks and bridges, confirming the answer.