To solve this problem, we need to determine the coordination number of each ion in the compound AB.$\newline$The coordination number is influenced by the ratio of the ionic radii of the cation and anion.$\newline$Given:$\newline$• Ionic radius of $A^{+} = 0.98 \times 10^{-10} \,$m$\newline$• Ionic radius of $B^{-} = 1.81 \times 10^{-10} \,$m$\newline$The radius ratio is calculated as follows:$\newline$Radius ratio $= \frac{Radius \ of \ A^{+}}{Radius \ of \ B^{-}} \newline = \frac{0.98 \times 10^{-10}}{1.81 \times 10^{-10}} \newline = \frac{0.98}{1.81} \newline \approx 0.541 \newline$The radius ratio helps determine the coordination number:$\newline$• If the radius ratio is between 0.414 and 0.732, the coordination number is 6.$\newline$• If the radius ratio is between 0.225 and 0.414, the coordination number is 4.$\newline$Since the calculated radius ratio is approximately 0.541, it falls within the range for a coordination number of 6.$\newline$Therefore, the coordination number of each ion in AB is 6.$\newline$This corresponds to Option 1.