Explanation

(d)
Atomic size increases in a group from top to bottom. But in IIIA group, gallium (Ga,1.35 $\mathring{\mathrm{A}}$) has size smaller than aluminium (1.43 $\mathring{\mathrm{A}}$). The reason is that in gallium $d$-electrons shield nuclear charge poorly and hence, due to greater effective nuclear charge $(Z_{eff})$ it has smaller size.