Which theorist claimed that people rise to their proper level in society based solely on their belief in a meritocracy?

That sounds like Social Darwinism so it would be Charles Darwin.

Urban growth invariably feeds increased commercialization and industrialization. it requires continually expanding land areas. when urbanization rates are high, and countries do not have the institutional and managerial capacities or economic wherewithal to manage such rapid growths, they exert intense pressure on the types and nature of the resulting settlements. they contribute to the development of overcrowded slums and shanty towns without adequate essential services and necessary infrastructure.-by asit k. biswas and cecilia tortajada