In a binary compound, atoms of element A form a hcp structure and those of element M occupy \( \frac{2}{3} \) of the tetrahedral voids of the hcp structure. The formula of the binary compound is:
\( \mathrm{M_4A} \)
\( \mathrm{MA_3} \)
\( \mathrm{M_2A_3} \)
\( \mathrm{M_4A_3} \)