In ecology, the size of a population is technically called:

The correct answer is Option (1) → Population density

The size of a population tells us about its status in the habitat. In ecology, population size has a technical name, population density, written as N. It need not always be a count: biomass or per cent cover can also be used.

Option (1) Population density — Correct. This is the technical name for population size (N). Any change in a population, from competition, predation or a pesticide, is judged by the change in N.

Option (2) Population growth — Incorrect. Population growth is the change in population size over time, not the size itself.

Option (3) Carrying capacity — Incorrect. Carrying capacity (K) is the maximum number a habitat’s resources can support, not the size of the population at a given time.

Option (4) Natality — Incorrect. Natality is the number of births during a given period. It adds to the population size but is not the size itself.

NCERT Extract:
Population Attributes – “… The size of the population tells us a lot about its status in the habitat. Whatever ecological processes we wish to investigate in a population, be it the outcome of competition with another species, the impact of a predator or the effect of a pesticide application, we always evaluate them in terms of any change in the population size. … Population size, technically called population density (designated as N), need not necessarily be measured in numbers only.”