Answer:
B. Guilt
Step-by-step explanation:
The sound of the heart beating symbolized the guilt of the narrator. He had just killed the old man and the officers were in the house searching. Although things went well, he imagined the sound of the heart of the old man beating under the floorboards and it grew louder and louder as the officers were chatting with him. The narrator eventually succumbed to the loud beat and confessed his crime because he could no longer take the guilt building up inside of him.