Which of the following is a way for a state to achieve international sovereignty?
a.
declare sovereignty in a document, such as the declaration of independence
b.
pass a law requiring other states to recognize its sovereignty
c.
gain recognition of sovereignty from the united nations
d.
merge with another state and form a new state

In order for a state to achieve international sovereignty, they must gain recognition from the United Nations (C).

Soverignty basically means that a nation is able to govern itself. While, sovereignty could be declared in a document, it isn't official until the rest of the world views it as an independent nation. Since it is impossible to pass a law requiring other states to recognize its sovereignty (you can only make laws for your country), it needs to be recognized by a group like the United Nations.