Final answer:
The Federal Flight Deck Officer (FFDO) is responsible for the security of the flight deck, using firearms if necessary to protect against criminal violence or air piracy.
Step-by-step explanation:
The primary duties and responsibilities of the Federal Flight Deck Officer (FFDO) are related to the security of the flight deck. FFDOs are trained and authorized by the United States federal government to use firearms to defend the cockpit of an aircraft against incidents of criminal violence or air piracy. Therefore, the most accurate description of the responsibilities of an FFDO is C) Security of the flight deck. Their role does not typically include ensuring passenger safety during turbulence, overseeing in-flight entertainment, or conducting pre-flight inspections as their primary responsibilities, although they are trained in various safety and security procedures that cover a broad scope of flight operations.