In a binary compound, atoms of element A form a hcp structure and those of element M occupy 2/3 of the tetrahedral voids of the hcp structure. The formula o the binary compound is:

Effective atom count in a hcp unit cell = 6 
So, A = 6
12 tetrahedral voids total in the hcp unit cell.
$M=\frac{2}{3}\times 12=8$
Consequently, $A_6{M}_8\text{ or }{A}_3{M}_4\text{ or }{M}_4{A}_3$ would be the binary compound's formula.
 Therefore, the correct option is B M₄A₃