The first ionisation enthalpy of gallium is slightly higher than that of aluminium, although Ga lies below Al. The best reason is

1. Athe lanthanide contraction operates between Al and Ga
2. Bgallium has a larger atomic radius than aluminium
3. Cthe inert pair effect makes gallium more stable as $\mathrm{Ga}{}^{+}$
4. Dthe ten $3d$ electrons shield the nuclear charge poorly, so the effective nuclear charge on the $4p$ electron is higher