Grammatically, all are OK.
However, I would only use (a) and (c) when the point of reference is the present time. You should only use the past perfect tense, for example, ("had taught") when the point of reference is an earlier time than the present, such as when telling a story which is set in the past.
Ex: "The school burned down in 2012. My father had been teaching there until the year before."