I think correct answer is Mortality bcz it is the no.of death per thousand individual per year . It decrease the population size and its density.It is death rate.

Understanding Population Dynamics
Population dynamics involves the study of how and why populations change over time. Various factors influence population size, including birth rates (natality), death rates (mortality), migration, and emigration.
What is Emigration?
- Emigration refers to the act of leaving one's resident country to settle in another.
- When individuals emigrate, they are permanently leaving the population of their home country.
How Emigration Leads to Permanent Decrease
- Direct Impact: When a significant number of people emigrate, the total population of their home country decreases permanently as these individuals no longer contribute to the natality or labor force.
- Demographic Shifts: Emigration can lead to demographic imbalances, such as a higher proportion of elderly individuals remaining in the country, which can further reduce the birth rate over time.
Comparison with Other Factors
- Migration: While migration includes both emigration and immigration, it does not inherently lead to a permanent decrease, as individuals may return.
- Natality: This refers to the birth rate in a population. A decrease in natality can reduce population size, but it doesn't guarantee a permanent decrease.
- Mortality: While high mortality rates can lead to a decrease in population, it does not directly correlate to permanent population loss, as births may still occur.
Conclusion
In summary, emigration is the factor most directly responsible for a permanent decrease in population size, as it removes individuals from the population permanently, in contrast to other factors which may not result in a lasting decline.