Answer:
D. Have the narrator slip and reveal an unexpected fact.
Step-by-step explanation:
The writer can hint at the truth of a story told by an unreliable narrator by "having the narrator slip and reveal an unexpected fact."
Given that fact is information or knowledge known to be true after being proved. Hence, when an unreliable slip and reveal an unexpected fact, it contradicts his other lies in his narration, thereby revealing the actual truth in a story.