This is a sufficient assumption where we need to find the answer choice that would provide the assumption that would logically validate this argument.
The reasoning in the farmer's argument is as follows:
You can be rich/poor, honest/dishonest (no middle ground). All poor farmers = honest. Therefore, all rich farmers = dishonest.
This is questionable reasoning. Just because all poor farmers are honest, what's to say that you can't be a rich farmer who is also honest.
That's where answer choice A comes in. If you state that "every honest farmer is poor", then you remove the possibility that a rich, honest farmer can exist, thus making every rich farmer dishonest.