Answer: The population size that the environment can sustain.
Explanation:
The carrying capacity can be defined as the maximum number of individuals of the population of a species which an ecosystem can support and sustain in terms of availability of resources such as water, food, shelter and other needs. When the population of the species exceeds the carrying capacity of the ecosystem the population starts declining to maintain the number which a carrying capacity of the ecosystem can sustain. On the basis of the above information, The population size that the environment can sustain. is the correct option.