You can't demand someone to do something (at least not in the version of English that I speak). This eliminates B and E.
C is wrong because "their demand to bring back the original Coke formula" implies that the consumers were demanding that they themselves should be allowed to bring back the formula, which obviously isn't what's meant.
D looks fine to me. I have no idea why they think it's wrong (if you're saying that they do). To me, A is not incorrect but there is a slight sense that the "demanding that..." clause is "hanging in mid-air". The definite "who demanded" in D seems stronger to me. So, I'd go for D.