The text begins with a comparison between the economies between the northern and southern states, so it may seem that the right answer to your question is the "compare and contrast" option. However, the text goes on to say that these differences between economies were one of the causes that resulted in the war of secession. That is, the war of secession is the effect caused by the differences in the economic sector of the southern and northern states. For this reason, the correct alternative is "cause and effect."
