To determine this, lets use the atomic mass for each element to determine the corresponding number of moles for a given mass of 1 g.
Mo: 95.94 g/mol
1 g/95.94 g/mol = 0.01042 moles Mo
Se: 78.96 g/mol
1 g/78.96 g/mol = 0.01266 moles Se
Na: 22.99 g/mol
1 g/22.99 g/mol = 0.0435 moles Na
Br: 79.9 g/mol
1 g/79.9 g/mol = 0.0125 moles Br
Thus, the answer is Na.
