The line "It was the owl that shrieked, the fatal bellman, / Which gives the sternest good night" suggests a specific interpretation of the owl's call as a harbinger of death. In this context, the owl represents a symbol of imminent doom, often associated with bad omens or death. Lady Macbeth interprets the owl's shriek as a signal that their dark deed (the murder of King Duncan) is underway.
Among the interpretations you provided, the most fitting one would be:
Lady Macbeth is beginning to lose her sanity and only thinks she hears an owl shrieking.
This interpretation holds weight because Lady Macbeth's initial composure begins to unravel as the gravity of their actions sets in. The imagery of the shrieking owl can illustrate her mental state, teetering between boldness and the dawning realization of what they have done. While she is currently driven by ambition and a sense of urgency, the reference to the owl serves as a reminder of the morally unsettling act they are committing, foreshadowing the psychological consequences that will follow.