The ionic radii of $A^{+}$ and $B^{-}$ ions are $0.98\times {10}^{-10}m$ and $1.81\times {10}^{-10}m$. The co-ordination number of each ion in AB is

The radius ratio, $=\frac{r_{A^{-}}}{r_{B-}}=\frac{0.98\times {10}^{-10}}{1.81\times {10}^{-10}}=0.54$
This is in between 0.414-0.732, which corresponds to octahedral geometry with coordination number 6.