The correct answer is - has a system of taxation.
Every nation that is sovereign has a system of taxation, they do differ from one country to another, but is something that is a must so that the country can function properly. The country is not obliged to be part of the United Nations, but is very useful if it is. It also doesn't mean that it has to obey another country, if that was the case, than there's no need or point of being sovereign. Most of the countries have outside influences, mainly from the world superpowers like USA, Russia, China, or regional powerful countries.