The correct answer is B: Gertrude is opening up the possibility that Ophelia did not kill herself. Since suicide was considered a mortal sin in Shakespeare's society, and those who killed themselves were barred from being buried in consacrated ground (something that will be elaborated upon by the gravediggers in Act V, scene I), the cause of her death had to be disguised. Here, Ophelia's fall into the river is attributed to distress and madness, and not to an act of will.