(a)
$H_{2}\underset{2000℃}{\overset{Electric\ arc}{\rightarrow}}2H\ \ - 104.5\ kcal$
hydrogen atomic
molecule hydrogen
The reaction is endothermic. For endothermic reaction increase in temperature shift the equilibrium in forward direction. To proceed forward the pressure must be low because for the above reaction, increase of pressure will favoured backward reaction. So, for maximum yield the conditions are high temperature and low pressure.