The correct answer is: a) Group together closely spaced towns.
An urban agglomeration is a continuous urban area consisting of a city and its surrounding towns, suburbs, and other closely settled areas. It is defined by the United Nations as a densely settled contiguous area comprising the city proper and other adjacent densely settled settlements that have a high degree of social and economic integration with the core city.
Urban agglomerations are important because they provide a measure of the size and growth of urban areas. They can also be used to identify areas that are experiencing rapid urbanization, which can have a significant impact on the environment and the economy.
Option b) is incorrect because rural-urban migration is the movement of people from rural areas to urban areas. It is a complex phenomenon that is driven by a variety of factors, including economic opportunity, education, and social change.
Option c) is incorrect because the expansion of cities is measured by the urban growth rate, which is the percentage change in the population of an urban area over a given period of time.
Option d) is incorrect because slums and informal settlements are not included in the definition of an urban agglomeration.