Final answer:
The staring, fat lady is the primary antagonist in "The Elevator" by William Sleator, symbolizing the protagonist's fears and internal struggles.
Step-by-step explanation:
The primary antagonist of "The Elevator" is d) the staring, fat lady. In this short story by William Sleator, Martin, the protagonist, faces his fears which are symbolized by the unsettling, silent presence of the fat lady who stares at him in the elevator. The story reveals themes of fear, insecurity, and the psychological effects of those fears. The fat lady serves as a personification of Martin's internal struggles, and while she may not do anything explicitly antagonistic, her imposing presence and the dread she invokes in Martin establish her as the central antagonist of the story.