Na in NaCl has 8 corners and 6 face atoms. If we remove face centered atom of one axis , two face atoms are removed.
Thus A is at 8 corners and B is at 4 faces.
$\therefore A = \large\frac{8}{8} = 1$
$\Rightarrow B = \large\frac{4}{2} = 2$
Thus formula is $AB_2$
Hence answer is (a)