The ionic radii of $A^{+}$ and $B^{-}$ ions are $0.98\times {10}^{-10}m$ and $1.81\times {10}^{-10}m$. The coordination number of each ion in AB is:
(NEET 2016)

$\text{Radius ratio of}\left(\frac{A^{+}}{B^{-}}\right)=\frac{0.98\times {10}^{-10}m}{1.81\times {10}^{-10}m}=\frac{0.98}{1.81}=0.541$

If the radius ratio is between 0.414 and 0.732 then, coordination number is 6.

Radius ratio	Type of void occupied	Coordination number
< 0.155	Linear void	2
0.155 to 0.225	Planar Triangular void	3
0.225 to 0.414	Tetrahedral void	4
0.414 to 0.732	Octahedral void	6
0.732 to 1	Body centred cubic void	8

Hence, option (a) is correct.