Final answer:
Lady Macbeth advises Macbeth to appear harmless and welcoming while secretly plotting King Duncan's murder, a reflection of the themes of appearance versus reality and ambition.
Step-by-step explanation:
When Lady Macbeth tells Macbeth to "Look like the innocent flower, but be the serpent under't," she is instructing him to present himself as harmless and welcoming (the innocent flower) while secretly harboring the deadly intent and cunning of a serpent. This metaphor emphasizes the need for deception to achieve their ambitious goal—to murder King Duncan—without raising suspicion. She encourages Macbeth to hide his dark intentions behind a façade of loyalty and kindness, reflecting the play's recurrent themes of appearance versus reality and the corrupting power of ambition.