Explanation

(d) The expansion of solids can be well understood by potential energy curve for two adjacent atoms in a crystalline solid as a function of their intermolecular separation $(r).$
At ordinary temperature: Each molecule of the solid vibrates about it’s equilibrium position $P_{1}$ between $A$ and $B$ ($r_{0}$ is the equilibrium distance of it from some other molecule)
At high temperature: Amplitude of vibration increases $(C \leftrightarrow D\ and\ E \leftrightarrow F).$ Due to asymmetry of the curve, the equilibrium positions $(P_{2}\ and\ P_{3})$ of molecule is displaced. Hence it’s distance from other molecules increses $\left( r_{2} > r_{1} > r_{0} \right).$
Thus, on raising the temperature, the average equilibrium between the molecules increases and the solid as a whole expands