The correct option is A Both have a positive effect on population density
Population density is the number of individuals present per unit area.
Natality (B) is the number of births during a given period that is added to the already existing population.
Immigration (I) refers to the number of individuals of the same species that move into the population, from elsewhere, during a given time period.
In both these processes, individuals are added to the population and hence it results in an increase in population density. If the population density at time t is Nt and the population density at time t+1 is Nt+1, then the effect of mortality and immigration on population density can be represented as -
Nt+1 = [Nt + (B+I)]
This shows that both natality and immigration have a positive effect on population density.