The correct answer is C) mutual defense of member nations.
Article 5 of the NATO Treaty promise to the mutual defense of member nations
"Restore and maintain the security of the North Atlantic area."—Article 5, NATO Treaty; April 4, 1949.
The North Atlantic Treaty Organization is a key strategic agreement, a military agreement indeed, signed by 30 European and North American countries to protect each other in case of military aggression or intervention.
The headquarters of NATO are located in the European city of Brussels, in Belgium. Among the countries that are part of NATO are the United States, Great Britain, Canada, France, Belgium, and Spain.