The observation that the king looked like her father suggests that Lady Macbeth is grappling with her emotions about the murder. The most fitting interpretation is:
a) She reveals the fact that she is not capable of murdering someone.
This interpretation highlights the internal conflict she experiences, as recognizing a familial resemblance could evoke feelings of guilt or remorse over the act of killing someone who resembles a loved one, indicating that her resolve may not be as strong as she initially believed.