Final answer:
The rumor in Pepys' diary about the cause of the fire was likely that it was an act of arson by Catholic conspirators during a time of religious and social tension.
Step-by-step explanation:
The rumor in Pepys' diary regarding the cause of the fire suggested that it was the result of arson committed by Catholic conspirators. This reflects a common theme in several historical accounts where catastrophic fires were often attributed to various scapegoats or incendiary motives, not uncommon during a period of religious and social tension. Some believed that Catholics were plotting to burn down Protestant cities and take control, and such fears were exacerbated by other incidents of the time, such as the Stono Rebellion.