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)