"Hacía varios días que alguien me acosaba" means:
Someone was stalking me for several days.
I was stalked for several days by someone.
Someone had stalked me for several days.
None of these answer carry the sense of being in the time frame of the past the way the examples do, such as "someone had been stalking me for several days". In English, I don't think the 2nd or 3rd answer are functionally any different. The first one is the only one to partly give a sense that this is an ongoing thing, even though it doesn't give the same frame of reference.
I suggest you change the available answers.