Terrorist attack is the modern cause of anxiety that often appears in invasion literature written in the United States today.
Terrorist attack may be defined as that sort of attack involving the deliberate use of violence against civilians in an attemp to attain political or religious aims.
Terrorism has increasingly swept the world, especially in countries such as the United States, which endured the most fatal terrorist attack on, September 11th, in 2001.
The best correction of the clause fragment is to C. delete the word "although." Without that word, you get a proper single independent sentence, and not a fragment.
