Which of the following is an example of a cross-sectional study?

A cross-sectional study involves observing a population at a single point in time or over a short period, allowing researchers to gather data on various factors simultaneously. The U.S. census serves as a prime example of a cross-sectional study because it collects demographic information, including age, gender, income, and occupation, from millions of individuals at one specific time. This snapshot provides valuable data for understanding the characteristics of the population without tracking changes over time.

In contrast, long-term health trials involve ongoing data collection over extended periods, which is characteristic of longitudinal studies rather than cross-sectional ones. Similarly, surveys conducted every year also fall under longitudinal research as they track changes and trends over time rather than providing a single moment's overview. Lastly, marital studies over several decades focus on changes and developments within relationships over time, which again showcases a longitudinal approach rather than capturing a population snapshot. Thus, the U.S. census accurately embodies the nature of a cross-sectional study.