What is carrying capacity? 

A)  the interaction of natality and mortality 
B)  the proportion of males to females 
C)  the maximum size of population that the environment will support 
D)  the reproductive rate

Carrying capacity refers to the maximum number of individuals that a particular environment can sustainably support, given its available resources and other constraints. This is influenced by factors such as food availability, habitat quality, disease, competition, and predation. It represents the upper limit of population size that can be maintained over the long-term without degrading the environment or causing population crashes. Therefore, option C is the correct answer.

Choice A is incorrect because the interaction of natality (birth rate) and mortality (death rate) determines the growth rate of a population, but it does not reflect the constraints of the environment.

Choice B is also incorrect because sex ratio (the proportion of males to females) can affect reproductive success and sexual selection, but it does not directly relate to carrying capacity.

Choice D is incorrect because reproductive rate alone does not consider the availability of resources and other limiting factors that affect the population size.