The statement that best describes the role of push and pull factors in migration is: "Push factors usually decrease a country's population, while pull factors increase it."
This accurately reflects the understanding that push factors (such as war, famine, or economic hardship) compel individuals to leave their home country, resulting in a decrease in that country's population. Conversely, pull factors (such as better job opportunities, safety, or improved living conditions) attract individuals to a new location, thus increasing that location's population.