Answer:
Low birth rates
Step-by-step explanation:
The cause of European countries having population decline can be attributed to LOW BIRTH RATES. This is evident in the fact that the recent statistical survey has shown that the total fertility rates in the majority of European countries are 1.55 per woman.
However, the ideal birth rate according to the United Nations Population Division for an optimum population should be at least 2.1 per woman.
Hence, in this case, the correct answer is "LOW BIRTH RATES."