The answer is D) diffusion! I found this answer by process of elimination. Here is how:
Immigration involves moving to another country this would bring your old country’s population down and your new country’s population up.
Emigration is leaving your country to go to a new one also so this would bring your old country’s population down and your new country’s population up.
Death rate is people dying which brings the population down. The higher a death rate the lower a population.
This leaves only diffusion! Good luck and have an amazing day!