Final answer:
Lady Macbeth responds confidently to Duncan's question, assuring him that they will not fail in their plan to assassinate him.
Step-by-step explanation:
When Duncan asks what will happen if they fail to assassinate him, Lady Macbeth responds by saying that they will not fail. She reassures Duncan by telling him that they know what they are doing and will not let anything stand in their way. Lady Macbeth's response is confident and determined, reflecting her ambition and desire for power.
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