Explanation:

- Urbanisation (Option 1): This refers to the growth of cities as people move in from rural areas, boosting city populations mainly through migration, but sometimes also due to natural population increase (births minus deaths).

- Pseudo urbanization (Option 2): This is when a city’s population grows rapidly, but without proper economic or infrastructure growth. It’s often due to migration, not always natural increase.

- Cultural mosaic (Option 3): This describes a mix of cultures coexisting in society, and isn't related to population growth reasons.

- Internal mobilization (Option 4): This term isn’t specifically used for city growth due to natural increase; it usually refers to people moving within a country.

The process of city population growing due to natural factors (like births) is still called urbanisation.

Correct Answer: Option 1 - Urbanisation