Final answer:
The makeup of the U.S. population in the early 1800s was most significantly changed by European immigration, which expanded the workforce and diversified the culture, especially increasing the Roman Catholic population.
Step-by-step explanation:
The makeup of the U.S. population in the early 1800s was most changed by C. immigration from Europe. This period witnessed a significant acceleration in the pace of immigration, as Europeans were attracted to the United States by the prospect of employment and land. This influx brought a diversity of new foods, customs, religions, and particularly expanded the Roman Catholic population due to Irish and German immigrants.
Further, the market revolution connected local life with regional, national, and international markets, influencing the demographic changes occurring during this transformative period of United States history.