B. Increase the volume (in other words, decrease the pressure).
A is wrong because as temperature decreases, the reaction will shift to the left as the system is endothermic (positive delta H).
C. is wrong because as O2 gas is added, the reaction will shift to the left in order to relieve the stress on the system.
D. is wrong because a catalyst will increase the rate of the reaction by decreasing the activation energy. It doesn't affect equilibrium.
B is correct because the left side has two moles of gas while the right side has three moles of gas. Thus, when pressure is decreased (volume is increased--they are inversely related), the system will shift in the direction with more moles of gas.
The answer is Air and Gravity
