The correct answer is A) an international effort.
The global response to terrorism has been an international effort.
Not limited trough the United Nations, the response to terrorism has been an international effort in which most countries of the world are aware of the importance of preventing attacks and share information with other countries about possible terrorist cells. Not only the United States has been the target of terrorist attacks. European countries such as France, England, Belgium, and Norway have had terrorist attacks and they know how difficult it is to prevent the attacks and maintain national security inside their borders. That is why the global response to terrorism has been an international effort and continues to be that way in order to prevent more attacks.