Final answer:
The population size of a species that the environment can sustainably support is known as its carrying capacity. This is a variable limit determined by available resources and environmental conditions.
Step-by-step explanation:
The correct answer to the question is c) Carrying capacity. The concept of carrying capacity refers to the largest population size of a species that can be supported by the available resources in a habitat without harming the environment. It is a balance between the availability of life-sustaining resources such as food, water, and habitat, and the growth of the population. It's important to note that carrying capacity is not fixed and can vary due to changes in resources, technology, and environmental conditions.