This is an odd one. If the sentence was worded as "Margaret and Jane", then the correct answer would be B. However, "as well as Jane" is an auxiliary statement. Therefore, within the sense, they are NOT considered to be part of the same group, and thus you cannot exchange their names for "they" (the pronoun which would make B correct). It's a weird oddity of sentences like that that both women should be treated as individuals, and thus "has" is correct. More specifically, due to how the sentence is written, you cannot rewrite it as "Margaret and Jane have been here since last week", despite that being the case. You would instead write it as two sentences: "Margaret has been here since last week. Jane has also been here since last week." By doing this, it becomes clear that A should be chosen in this very specific instance.

M & J have been. They have been. 3rd person plural => have I have, you have he has, we have, you (pl) have, they have.

Thanks! :D

Glad that you can read the list. Hope you will achieve your goal. Remember you will gain for every effort you pay. Keep it up!

Hi Apple Tree, thanks for this comprehensive list. It is very useful =)