A population 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.

The correct answer is that a population overshoots the carrying capacity and environmental pressures and shortfalls begin to exert their effects. A certain area of environment only has limited resources such as food, water, habitat, etc. These limited resources are called the carrying capacity of the environment. If the population exceeds the carrying capacity of the environment, there will be not enough resources to sustain the population therefore members of the population will die and the overall population number will crash hence the term population crash.