Final answer:
In 'Macbeth,' Lady Macbeth is the character who says "What's done cannot be undone," showing her despair and guilt over their actions.
Step-by-step explanation:
The famous line "What's done cannot be undone" is spoken by: b) Lady Macbeth. This poignant statement is found in Act 5, Scene 1 of William Shakespeare's "Macbeth." Lady Macbeth utters these words as she sleepwalks, revealing her guilt and remorse over the atrocities committed by herself and Macbeth. The line reflects the irreversible consequences of their actions and the psychological burden they carry. Lady Macbeth's descent into madness and her inability to escape the weight of guilt contribute to the tragic atmosphere of the play. This line is a powerful reminder of the enduring impact of moral transgressions in the unfolding tragedy of "Macbeth."