Which of the following country is not a member of Collective Security Treaty Organization (CSTO)?

Q. Which of the following country is not a member of Collective Security Treaty Organization (CSTO)?
(A) Armenia
(B) Kyrgyzstan
(C) Tajikistan
(D) Azerbaijan

The answer is: (D) Azerbaijan
The six members of Collective Security Treaty Organization (CSTO) are: Armenia, Belarus, Kazakhstan, Kyrgyzstan, Russia, and Tajikistan.