The most fitting interpretation of the owl that Lady Macbeth claims to hear is: Lady Macbeth is beginning to lose her sanity and only thinks she hears an owl shrieking.
This interpretation aligns with the themes of guilt, madness, and the psychological consequences of their actions throughout the play, particularly as the tension and moral implications of their deeds weigh heavily on them. The owl, often associated with death, symbolizes the darkness of their actions and the unsettling mental state of the characters.