It seems that you have missed the given options for this question which are the following: a) the increase in religious-based sex-abstinence programs
b) the availability and use of effective contraceptives
c) the decrease in the number of people getting married
d) an increase in same-sex couplings
Based on this, the reason that is most likely why the dramatic decrease in the number of children in the average American family is the the availability and use of effective contraceptives.