The only possible answer to this question is A.
First, you should note that A and B directly contradict one another. It is important to notice this sort of thing as a test-taking strategy. This tells you that "All of the above" cannot be right, narrowing your choices to three.
Another hint is that B and C are similar. "Expansion" and "increased in number" mean very similar things. This should make you suspicious.
Finally, your knowledge of the 1920s should tell you that A is correct. It may be a little overstated to say that farmers were the "only" group not to experience prosperity. However, it is clearly correct to say that farmers did not prosper during this decade. The farm sector experienced tremendous difficulties even as much of the rest of the economy boomed.