The primary reason for the shrinking population in certain parts of Europe is primarily C: The fertility rate is low. Many European countries have fertility rates that fall below the replacement level, which means there are not enough births to sustain the population size.
While immigration levels and migration patterns can influence population dynamics, the low fertility rate is a significant factor contributing to the overall population decline in some European nations. Additionally, the aging population and higher death rates among older adults can also play a role, but the low birth rate is the most consistent and impactful reason.