Apopulation crash occurs when a. environmental resistance comes into play gradually. b. the population growth rate slows. c. resources are essentially unlimited. d. a population overshoots the carrying capacity and environmental pressures and shortfalls begin to exert their effects. e. a population approaches its carrying capacity.

d. a population overshoots the carrying capacity and environmental pressures and shortfalls begin to exert their effects.

Explanation:

A carrying capacity of the ecosystem can be define as the maximum size of a population that an ecosystem can support in terms of availability of resources such as water, food, mates and other requirements. The population crash occurs when the population exceeds the carrying capacity of the ecosystem.  In such situation an overshoot of the population occurs when the birth rate exceeds the death rate or when the rate of emigration is more than the rate of immigration. As, a result of this the ecosystem will be incapable of supporting the exceed in numbers in terms of resources. Then the population declines by the effect of diseases, predation, natural calamities and by other shortfalls.