Final answer:
The number of married adults in the U.S. is currently at an all-time low, with a significant decrease in married households since 1960 and changing societal attitudes towards marriage and cohabitation.
Step-by-step explanation:
The number of married adults in the United States has changed significantly over time. Historical data and trends suggest that the correct answer to the question is: C) Is at an all-time low. Since 1960, there has been a noticeable decline in the percentage of U.S. households headed by married couples. For instance, in 1960, 66 percent of households were headed by married couples, which dropped to 45 percent by 2010. Moreover, there is a growing trend of young adults choosing to delay marriage or opting out of it entirely. The rise in cohabitation and single-parent households, as well as the increasing acceptance of non-traditional family structures including same-sex couples and expanded households, corroborates this shift. Furthermore, high rates of divorces following the liberalization of divorce laws and societal changes, such as an increase in the workforce participation of women, have influenced these numbers, although recent decades have seen a stabilization of divorce rates due to factors such as older age at first marriage and higher education levels amongst married individuals.