I'm unable to view images, but I can provide some guidance on how to determine which industry's decline would hurt the southern economy the most based on common knowledge about the southern economy.
Generally, agriculture has been the backbone of the southern economy, especially during periods of reliance on cash crops like cotton, tobacco, and sugar. While iron production, manufacturing, and railroad construction contribute to the economy, agriculture typically has a more significant impact on the overall economic stability and employment in the South.
If you analyze the graph you have, look for indicators such as employment numbers, revenue contributions, or overall economic impact. The industry that shows the most significant economic involvement would likely be the one whose decline would hurt the southern economy the most.
If the graph indicates that agriculture is the largest contributor, you would choose "agriculture." If another industry has a more striking impact, then that would be your answer.