At the end of the Reconstruction era that followed the American Civil War, tens of thousands of Black former slaves left the southern United States—in a movement known as the Great Exodus—for finding social and economical opportunities in the Western states and territories.A) At the end of the Reconstruction era that followed the American Civil War, tens of thousands of Black former slaves left the southern United States—in a movement known as the Great Exodus—for finding social and economical opportunities in the Western states and territories. -> Okay..It's nice to have modifiers in place. But, only error, I found "for finding". Incorrect
B) In a movement known as the Great Exodus at the end of the Reconstruction era that followed the American Civil War, tens of thousands of Black former slaves left the southern United States—to find social and economical opportunities in the Western states and territories. -> we need "In a movement known as the Great Exodus" after " tens of thousands...." Incorrect.
C) At the end of the Reconstruction era that followed the American Civil War, in a movement known as the Great Exodus, tens of thousands of Black former slaves left the southern United States for finding social and economic opportunities in the Western states and territories. -> Modifier error. We need "tens of thousands of Black former slaves..." after ".......followed the American Civil War," .Incorrect.
D) To find social and economic opportunities in the Western states and territories, tens of thousands of Black former slaves left the southern United States at the end of the Reconstruction era that followed the American Civil War, in a movement known as the Great Exodus. -> Meaning changed. We don't have events in sequence. Incorrect.
E) At the end of the Reconstruction era that followed the American Civil War, tens of thousands of Black former slaves left the southern United States—in a movement known as the Great Exodus—to find social and economic opportunities in the Western states and territories. -> It is better. first, at the end of the Reconstruction era, followed by Civil war, then next movement of people, it was called something and finally reason of people movement.
So, I think E.