The metallic radius of aluminium is $143\mathrm{pm}$ and that of gallium is $135\mathrm{pm}$, although gallium lies one period below. This is best explained by:

1. Agallium having fewer electron shells than aluminium
2. Bpoor shielding by the ten $3d$ electrons inserted before gallium, giving a higher $Z_{\mathrm{eff}}$ than simple extrapolation predicts
3. Cthe lanthanoid contraction, which acts on all elements of Group 13
4. Dthe inert pair effect making gallium use fewer bonding electrons