Citizenship is best defined as a member of a state who is legally and constitutionally

Citizenship is best defined as a member of a state who is legally and constitutionally recognised. A citizen is a person who has been formally acknowledged by the laws and constitution of a country as belonging to that state, thereby enjoying certain rights (such as the right to vote, right to hold public office, and right to a passport) while also bearing obligations (such as paying taxes and obeying the law).

The word "recognised" captures the essential legal relationship between a person and the state. Citizenship is conferred through specific legal mechanisms - birth (jus soli or jus sanguinis), registration, or naturalisation - and once a person is recognised as a citizen, the state acknowledges their membership and the rights that come with it.

The other options do not accurately complete the definition. "Established" implies setting something up or founding it, which does not describe the relationship between a citizen and the state. "Associated" suggests a loose or informal connection, which is too weak to describe the binding legal relationship of citizenship. "United" implies merging or joining together, which does not capture the nature of legal recognition that defines citizenship.