The salt bridge acts to maintain a constant ion concentration for both of the half-cells. The anions are moving toward the oxidation side and the cations are moving toward the reduction side via the salt bridge.
Therefore the answer is C- provide constant overall balanced concentration of ions.
Standard equation would be N2(g)+3H2(g)==>2NH3(g), so through stoichiometry, (4 mol N2)(2mol NH3/1 mol N2), assuming excess H2, would yield 8 moles of NH3.
