Which factors could cause the largest increase in the effective population size of a species?

Answer: A reduction is mortality rate and an increase in the number of reproductively active male and female specie of the population.

Explanation: For an increase in population size to occur among a certain animal specie, there needs to be a reduction in the death rate (mortality rate) of that specie and an increase in birth rate in that specie which arises as a result of an increase in the number of reproductively active male and female population in the specie.