Answer: D
Partial pressures: pNX2OX4=2.0×1/4=0.5 atm and pNOX2=2.0×3/4=1.5 atm. Qp=pNOX22/pNX2OX4=(1.5)2/0.5=4.5 atm, which exceeds Kp=3.0 atm, so the reaction goes backward. Using mole fractions in place of pressures gives 2.25 and the wrong direction.