The 2001 terrorist attacks show that a general weakness in u. s. intelligence gathering activities exists in the area of intelligence.

I believe the answer is: Human
Area of human intelligence in this context refers to predicting the behavior that displayed by people who come in and out from the united states.
One effort that united states created after the terrorist attack to handle this was the creation of TSA to increase security in the airport.

Ithink it is b  employees