Final answer:
The 9/11 terrorist attacks led to FBI agents carrying guns and making arrests, increasing their law enforcement role.
Step-by-step explanation:
The event that led to FBI agents carrying guns and making arrests, increasing their law enforcement role, was c) The 9/11 terrorist attacks. After the attacks, the FBI's focus shifted significantly towards counterterrorism efforts, and they were granted more authority and resources to combat terrorism. This included the carrying of firearms by agents and the ability to make arrests in relation to terrorist activities.