Answer:
The correct answer would be a mass extinction.
Decreased population size greatly reduces the genetic variation of the population.
It makes the population susceptible to the extinction.
It makes the population vulnerable to the disease.
The entire population can be extinct if a change in an environment takes place.
For example, in a bacterial population, if a single bacteria is vulnerable to one antibiotic then the entire population is vulnerable to the same antibiotic due to the identical genome.
Thus, reduce population size can cause mass extinction.