The answer is A, cause/effect. Marcia is focusing on how the war started, what ended it, and how it affected America. Cause/effect covers all these things, while the other answers either skip over these things completely or concentrate on only a few.
Answer:
The main idea is how sad it was to leave the farm.
Explanation:
The narrator goes on to tell us that people were crying, how much it was a horrible experience, and that there was many arguments. Although it talks about other things towards the end, the central idea is how sad it was to leave the farm.
