Final answer:
The witches' prophecies implant the idea of the evil deed in Macbeth's mind, triggering his existing ambitions rather than providing an actual poison for King Duncan's murder.
Step-by-step explanation:
The witches do not actually provide Macbeth with a charm to use to poison the king. Instead, the prophecies given by the witches serve to implant the evil deed in Macbeth's mind (Option C). These prophecies tap into Macbeth's existing ambitions and set off a chain of events leading to King Duncan's murder, orchestrated by Macbeth and Lady Macbeth without any supernatural poison. Lady Macbeth's plan involves getting the king's guards, so that they are unable to prevent or witness the murder, rather than using a poison.