Gravitational Potential Energy

Gravitational potential energy near the Earth's surface is measured relative to a chosen reference level and changes by $$\begin{gathered} \Delta \\ cup=mgh \end{gathered}$$ when a body is raised through a vertical height $h$. Because the books are lifted slowly at constant speed, all the work done by the librarian against gravity is stored as potential energy with no change in kinetic energy. With $m=5\text{ kg}$, $g=10{\text{ m/s}}^2$, and $h=1.8\text{ m}$, the increase is $$\begin{gathered} \Delta \\ cup=5\times 10\times 1.8=90\text{ J} \end{gathered}$$. The option 9 J drops a factor of ten through a height misreading. The option 45 J uses half the height for no reason. The option 180 J doubles the height, perhaps confusing diameter with radius-like reasoning. This matches the NCERT treatment of potential energy as path-independent work done against gravity. As a sanity check, the units $\text{kg}\times {\text{m/s}}^2\times \text{m}$ reduce to joules, and a modest shelf lift storing about 90 J is reasonable.