14) D

A – Wrong
The addition of a catalyst never affects the equilibrium of a reaction.

B – Wrong
Number of moles of gas on left side = Number of moles of gas on right side. Hence, decreasing pressure will produce no change.

C – Wrong
Number of moles of gas on left side = Number of moles of gas on right side. Hence, decreasing pressure will produce no change.

D – Correct
Decreasing temperature will favour the exothermic reaction (forward reaction which reduces amount of CO)

E – Wrong
No nitrogen is involved so it would have no effect on this reaction.