Final answer:
The growth of the Muslim population in Western Europe is mainly due to immigration from predominantly Muslim countries, coupled with the declining family size among native Europeans.
Step-by-step explanation:
The main factor contributing to the growth of the Muslim population in Western Europe is immigration from predominantly Muslim countries in the Middle East and North Africa.
Despite the fact that Muslims make up less than 10 percent of the total population in some Western European countries, Islam is reported to be the fastest-growing religion in the region.
This demographic shift has been influenced to a significant degree by economic factors, as well as the declining family size among native Europeans due to various reasons, such as the high cost of living, delayed marriage and childbirth, and the prevalence of career-oriented lifestyles among women.