The correct answer is C.
Better health facilities should lead to better caring during pregnancy, at labouring, and for the early years of the baby once he/she is born. Health systems in highly-developed contries have allowed to achieve extremely low number of natural abortions and mortality rates for babies and infants and also for mothers at giving birth. Therefore, in terms of healthcare, conditions are perfect to favour higher birth rates than in previous times, with worse health systems. But the trend is actually the opposite, because birth rates keep on decreasing.
Therefore the factors generating this trend are mainly economic and social, such as the ones described in the options above. For example, letter A. Families in the past needed to have a lot of children so that they could help with agricultural work to be able to feed the whole family. This is not the case anymore in highly-developed societies. It is also true what letter B states. Women have entered the workforce and attend superior studies therefore they usually decide to start a family at a later age, so this means a shorter par of lifetime dedicated to have children. And this is also connected to the cause in letter D. Due to the participation of women in the laborforce since the 20th century, it is a new phenomenon that in fact there are women who choose not to have children in order to dedicate all their efforts to their proffesional careers.