It's just another poor question, in my opinion. It is hard to understand exactly what is going on (is the client being accused of making valid transactions?) or the pupose of B's statement. Both (A) and (B) could fit, I suppose. If forced to make a choice I would probably go with (A).
Sentence B is actually two sentences that cannot be joined by any of the choices,. You need a period followed by a capital letter. instead of the first comma.
In terms of meaning, none of the choices makes good sense to me..
Finally, illegitimate/illicit ar not good vocabulary choices here. Maybe invalid/illegal.