The correct order of the first ionization enthalpy is:

🧠 d-block contraction keeps Ga's IE close to Al Down Group 13, IE normally drops. But Ga follows the filling of the 3d subshell. The 3d electrons shield poorly, so the effective nuclear charge on Ga's valence electrons is higher than expected. This makes Ga's IE nearly the same as Al's (actually slightly higher). 🗺️ The ranking B ($Z=5$, smallest size) has the highest IE. Then the d-block contraction effect means Ga ($IE = 579\,\text{kJ mol}^{-1}$) is slightly above Al ($IE = 577\,\text{kJ mol}^{-1}$). So: $\ce{B} > \ce{Ga} > \ce{Al}$.

The correct is $\ce{B} > \ce{Ga} > \ce{Al}$

⚠️ Common mistake Many students assume IE always decreases smoothly down a group. The 3d contraction in Ga breaks this.

$\boxed{\text{Answer: (d)}}$