The correct option is D population density
Population gradient :
Population gradient is the change in the density of a population of a given area over a defined space.
Population census :
Population census refers to the official enumeration of population strength in a defined territory such as a country. It also involves data collection on selected demographic and socioeconomic features of the population.
Population pressure :
Population pressure refers to the stress caused by the growing population on the environment which in turn reduces the ability of an environment to support the population.
Population density :
Population density refers to the number of individuals present within a given area. Population size refers to the number of individuals in a population. Population size technically is population density.
Final answer :
(D) Population density