Urban Growth is the term that denotes the net population increase of towns and cities.

Explanation:
Urban Growth refers to the increase in the population and physical extent of urban areas over time. It is often measured in terms of the net population increase, which is the difference between the number of people moving into urban areas and the number of people moving out. Urban growth is influenced by various factors such as migration, natural increase (births minus deaths), and changes in administrative boundaries.

Let's break down the options to understand why Urban Growth is the correct term for the net population increase of towns and cities:

a) Urban Growth: This term specifically refers to the increase in population and physical extent of urban areas. It encompasses both the population increase and the expansion of urban areas.

b) Population Growth: While population growth is a broader term that can be applied to any area, it does not specifically denote the net population increase of towns and cities. It includes population changes in rural areas as well.

c) Urbanisation: Urbanisation refers to the process of the population shifting from rural to urban areas. While it is related to urban growth, it does not directly represent the net population increase of towns and cities.

d) Urban Area: This term refers to a geographic area with a high population density and a significant amount of built infrastructure. It does not specifically denote the net population increase but rather describes an area that satisfies certain urban characteristics.

In conclusion, Urban Growth is the term that specifically denotes the net population increase of towns and cities. It encompasses both the increase in population and the expansion of urban areas.