My impression is that it should be, "She is visiting her grandmother this Saturday evening." If you have a set time and place, and other people know about the arrangement, then you use present progressive plus a future time. Unfortunately, "I know her plan" is really misleading. Does her grandmother know about the visit? The sentence doesn't tell us (and yes, we need this piece of information); only that the speaker knows about the visit. Also, "plan" is one of the keywords used when teaching the "going to" rule. The keyword with present progressive for future is "arrangement". I think the writer of the question wanted to give a clear hint, but it was clumsily done. Unfortunately, this makes the question unclear.