The middle classes are moving out of Mumbai to Navi Mumbai because the city has become too congested. Navi Mumbai has larger buildings with floor sizes, and affordable prices. The land is planned out with entertainment zones and shopping areas.

Urban sprawl is a phenomenon where urban areas expand into the surrounding rural areas, often driven by the need for more affordable housing and better living conditions. In the context of Maharashtra, particularly around the city of Mumbai, urban sprawl has led to the development of new suburbs like Navi Mumbai.

Navi Mumbai is a planned suburb that was developed to accommodate the growing population of Mumbai. The city of Mumbai has become increasingly congested, with high property prices and limited space for expansion. As a result, the middle classes have started to move out of Mumbai to Navi Mumbai, which offers larger buildings with more floor space and more affordable prices.

The development of Navi Mumbai is a planned urban development project that aims to provide better living conditions for the residents. The city has been designed with planned entertainment zones and shopping areas, making it an attractive option for those looking to move out of the congested city of Mumbai.

It is important to note that Navi Mumbai is not a slum or a rural area. Slums typically develop due to unplanned urbanization and lack of proper infrastructure, which is not the case for Navi Mumbai. Rural areas are characterized by agricultural activities and low population density, which does not apply to Navi Mumbai.

In conclusion, the development of Navi Mumbai as a suburb is driven by the need to accommodate the growing population of Mumbai and to provide a more livable environment with larger buildings, affordable prices, and planned entertainment and shopping zones. This makes Option C the correct answer, as it accurately describes the development of Navi Mumbai as a suburb.