Correct Option: D

Central Place Theory (Christaller, 1933)
Christaller’s model is based on the following assumptions that every part of the plain is served by a central place and the circles of influence of these central places fills the plain. A central place is a place offering a variety of goods in the centre of the market area, such as a village, town or city. In the K = 3 system or market principle optimises the markets of each central place. Supposing a region has three cities, then it will have 9 towns and 27 villages, and so on.

Core Periphery Model, is a concept where Core regions are based on comparative advantages-resource endowment and location. Periphery is inaccessible, under populated and has poor resources. Favourable effects flow from center to periphery.
The Rank size rule is an empirical regularity. The settlements within a defined area are ranked in descending order according to the size of their population. Urban primacy indicates the ratio of the largest city to the next largest i.e the second largest in a country.
Gravity model, as social scientists refer to the modified law of gravitation, takes into account the population size of two places and their distance. The gravity model of migration is a model in urban geography derived from Newton’s law of gravity, and used to predict the degree of migration interaction between two places.
Entropy is an important concept in the studies on complex systems such as cities. Spatial patterns and processes can be described with varied entropy functions. This model is a criterion to measure urban population distribution and the distribution of cities at urban classes in the region.