Final answer:
Lady Macbeth's mental decline is driven by her guilt and remorse over Duncan's murder, which overwhelms her and leads to her tragic fall. Hence, option (a) is correct.
Step-by-step explanation:
The mental decline of Lady Macbeth is primarily driven by deep-seated guilt and remorse following her role in King Duncan's murder. Her ambition pushes her to instigate and partake in the regicide, as seen in her influential and manipulative encouragement of Macbeth. However, as the play progresses, her initial steely demeanor gives way to a tormented conscience that is haunted by the weight of their crime.
Lady Macbeth's guilt manifests in her sleepwalking, wherein she tries to wash the 'filthy witness' of blood from her hands, revealing her psychological turmoil. Unlike Hamlet, whose emotion towards his mother Gertrude is 'in excess of the facts as they appear,' Lady Macbeth's decline is directly related to the consequences of her actions.
Her power and influence at court, alongside her partnership with Macbeth, crumble as her sanity wanes, underlining her tragic fall from a position of control to one of helpless despair.