Final answer:
Lady Macbeth referred to 'blood' when she complained about a spot on her hands, which symbolizes guilt from her part in the murderous acts.
Step-by-step explanation:
When Lady Macbeth complained about a spot on her hands, she was referring to blood. This is evident in various lines from Shakespeare's play Macbeth, where blood symbolizes guilt and murder.
For instance, Lady Macbeth tries to encourage her husband to wash the 'filthy witness' from his hands, referring to the bloodstains from their murderous act. Later, Lady Macbeth becomes tormented by guilt, visualizing blood on her hands that cannot be washed away, exclaiming that not even 'all great Neptune's ocean' could remove the bloodstains.