The answer is A. An aging and decreasing population is a demographic challenge that Europe is currently experiencing.
According to an article by the New York Times, written in 2008, the decrease in population was first noticed by european demographers in the 1990's, when evidence of a falling birthrate begin to appear, birthrate is the average number of births per woman.
Historically, birthrates tend to fall dramatically during wars or famines, however the fall that it has experienced during the last decades in developed countries obeys to different reasons. The shift of populations from rural areas to cities, and adopting urban lifestyles; higher living costs; and the incursion of women in the job market, are some of the reasons that could help explain this demographic tendency.