why is c wrong

It does indeed look quite similar, but it flips around the most, and we aren't allowed to flip most statements. We do know that the majority of the members work in that building. However, this is not the same as saying that most of the people in the building work in that unit. Under C, it would be possible that there are only 3 people in the building, and 2 of them are members of the unit. This doesn't guarantee that any of them are programmers, because it's possible that those two are among the non programmer members of the unit.