The main cause of the great migration to the United States in the late 1800s and early 1900s was a combination of factors, but among the options provided, "political unrest in Europe" and "religious persecution in Eastern Europe" were significant motivators for many migrants. However, if we focus on the broader context, "the need for workers in the U.S." also played a critical role in attracting immigrants.
So, while several options highlight important reasons, if you must choose one, I would suggest "political unrest in Europe" as a key catalyst for many who migrated during that period, especially from countries affected by conflict and government repression.