(c)
$N_{2}(g) + 3H_{2}(g) \rightleftharpoons 2NH_{3}(g);\ \$
$\ \ \ \mathrm{\Delta}H = - 93.6\ kJ\ mol^{- 1}$
This reaction is exothermic therefore the concentration of $NH_{3}$ can be increased by lowering the temperature, high pressure ($\mathrm{\Delta}n < 0$) excess $N_{2}\ and\ H_{2}$.