The most fitting interpretation of Lady Macbeth's observation that King Duncan resembled her father is B. She reveals the fact that she is not capable of murdering someone.
In this line, she acknowledges a moment of hesitation and moral conflict, indicating that the resemblance to her father evokes a sense of compassion or guilt, which ultimately prevents her from going through with the murder herself. This admission highlights her internal struggle and her recognition of the emotional weight of their actions.