The Nuclear Suppliers Group (NSG) is a multinational body concerned with reducing nuclear proliferation by controlling the export and re-transfer of materials that may be applicable to nuclear weapons development. It also aims to improve safeguards and protect existing materials.
Out of the given options, three countries—Russia, USA, and Italy—are members of the NSG, which significantly contributes to the international nuclear non-proliferation framework. Iran, however, is not a member of the NSG.
Therefore, the correct answer is:
Iran