The correct option is that only four of the listed countries are members of the North Atlantic Treaty Organization (NATO). Specifically, these countries are Bulgaria, Croatia, Montenegro (not listed), and Serbia is not a member. NATO, established in 1949 for collective defense against aggression, includes several Eastern European nations that joined post-Cold War to enhance security against potential threats from Russia. Austria and Sweden maintain a policy of military neutrality, while North Macedonia became the most recent member in 2020, reflecting NATO's continuous expansion to foster stability and defense collaboration in Europe. This information is relevant for understanding geopolitical alliances and security dynamics, crucial themes in UPSC examinations.