Which of the following is NOT a city-state?

A city-state is a sovereign entity that consists of an independent city and its surrounding territory, functioning as its own state. San Marino, Vatican City, and Athens all meet this definition; they are recognized as city-states with their own governance and political structures.

San Marino is one of the world's smallest countries and is entirely surrounded by Italy, governing itself independently. Vatican City, the spiritual and administrative center of the Roman Catholic Church, is also a fully sovereign city-state. Athens, in ancient times, was a powerful city-state known for its significant contributions to art, philosophy, and politics, particularly during the classical period of Greece.

In contrast, Istanbul does not qualify as a city-state. It is a major city in Turkey that serves as a cultural and economic hub, but it does not function as an independent sovereign state. Instead, it is part of a larger national framework, governed by the Turkish government. This distinction highlights why Istanbul is the correct answer to the question regarding which option is not a city-state.