The correct answer is option 2: Hong Kong. In 1997, Hong Kong became a part of China under the principle of `one country, two systems’. This principle was designed to allow regions like Hong Kong and Macau, which were previously under colonial rule, to maintain their own economic, legal, and administrative systems while being part of China.

Option 1: Tibet is not the correct answer. Tibet has been a part of China since the 1950s following the occupation by the People`s Republic of China. However, the `one country, two systems` principle does not apply to Tibet.

Option 3: Xinjiang is also not the correct answer. Xinjiang is an autonomous region in China, but it did not become a part of China under the `one country, two systems` principle.

Option 4: Inner Mongolia is also not the correct answer. Inner Mongolia is an autonomous region in China, but it did not become a part of China under the `one country, two systems` principle.

Therefore, it is clear that the correct answer is option 2: Hong Kong.