Final answer:
The correct answer is option a. Banquo's ghost is the character 'sitting' in Macbeth's chair at the feast, reflecting Macbeth's guilt and paranoia.
Step-by-step explanation:
In William Shakespeare's play Macbeth, the character who is "sitting" in Macbeth's chair when he returns to the feast is A. Banquo's ghost. This occurs during the infamous banquet scene where Macbeth sees the ghost of Banquo sitting in his place.
The appearance of the ghost is a manifestation of Macbeth's guilt over having ordered the murder of his friend Banquo. Macbeth's fear and paranoia are on full display during the scene, as he is the only one who can see Banquo's spectral presence, causing alarm and confusion among the other guests.