The correct answer is D.
Answers A, B and C are incorrect.
Your understanding of Le Chatelier's principle is correct; however, I think you simply misstated it. It will NOT REMAIN at equilibrium. You are right that if a reactant is added to the system which is at equilibrium then the system will shift to the right in order to reach a new equilibrium in which the concentrations of reactants and products have changed in order to keep Keq constant. The B answer is incorrect. It will NOT REMAIN at equilibrium but it will SHIFT to form a NEW equilibrium. I think your problem is with the word "remain". That implies that there will be no change. Yes, the system will "remain" at equilibrium but not before it shifts to reach a new equilibrium in which reactants and products have changed so that Keq is constant. Answer B clearly says that no change will occur. Had B said that I suspect you would not have picked that answer.