Q.Which of the following is not a member of Group III A or 1B?

Group 13 (B, Al, Ga, In, Tl) all share the outer configuration $ns^2np^1$, but their inner cores differ sharply: B and Al have simple noble-gas cores, while Ga, In and Tl carry filled d (and, for Tl, f) subshells that shield the nucleus poorly. This single structural fact explains the group's famous irregularities — gallium's atomic radius (135 pm) dips BELOW aluminium's (143 pm) despite having an extra shell; ionisation enthalpy jumps upward at Al→Ga and In→Tl instead of falling smoothly; and electronegativity, after falling from B to Al, edges back up for Ga, In, Tl rather than continuing to decrease. Boron's very high combined first-three ionisation enthalpy prevents it from ever forming a bare $B^{3+}$ ion, so boron is exclusively covalent; aluminium's much lower combined ionisation cost lets it readily form $Al^{3+}$ and behave as a strongly electropositive metal. Further down the group, the inert-pair effect makes the $ns^2$ electron pair increa …