The correct option is B N2(g)+O2(g)⇌2NO(g) ΔrH=−43200 cal
If ΔH>0 then endothermic
If ΔH<0 then exothermic
In exothermic reaction, heat is liberated in products side. There is an increase in temperature as we go in the forward direction. Similarly for an endothermic reaction, there is a decrease in temperature in the forward direction.
On decreasing temperature, forward reaction will be favoured if there is an increase in temperature for that side. It happens for an exothermic reaction.
Only option B is an exothermic reaction because it has ΔH<0.