Which country is not the member of NATO?

Which country is not the member of NATO?

A. Estonia
B. Netherlands
C. Norway
D. Finland 

Six EU member states, all who have declared their non-alignment with military alliances, are not NATO members: Austria, Cyprus, Finland, Ireland, Malta, and Sweden. Additionally, Switzerland, which is surrounded by the EU, has also maintained their neutrality by remaining a non-EU-member.