If you're having trouble deciding between (A) and (E), here's a very subtle hint.
Think about the grammar.
In terms of grammar, (A) is subject dense and (E) is predicate dense. In other words, the subject of (A) is heavily modified whereas the predicate of (E) is heavily modified.
The main subject of (A) is farmers. All farmers? No. It zooms in onto a subset of all farmers called farmers in the region. All of those? No. It further zooms in onto a subset of all farmers in the region who abandon the use of chemical fertilizers. All of those? No. It further zooms into a subset of those called "most". Okay, now that we've finally got the right zoom level, those farmers, what about them? What's the predicate? Well, just that they will periodically grow alfalfa.
Contrast that with (E). The subject is "some farmers in the region". In other words, at least one farmer in the region. What about them? (What's the predicate?) It's complicated. It's a conditional predicate. We're saying for at least one farmer in the region, the follow is true: if they grow green-manure crops, then they abandon the use of chemical fertilizers. In other words - remember your lawgic / translation lessons - their growing green-manure crops depends on their abandoning their use of chemical fertilizers.
Can you negate this statement?
When I say for some people, X is true, the negation of that is for no person is X true. (Review this lesson.) And that means for all persons, X is not true. (No dogs like to eat salmon = all dogs do not like to eat salmon.) So apply that here. For no farmer in the region is growing green-manure crops dependent on their abandoning use of chemical fertilizers. That means for all farmers in the region, not[growing green-manure crops dependent on their abandoning use of chemical fertilizers].
Remember how to negate conditional statements? Negated, the statement is that growing green-manure crops can happen alongside with not abandoning (that means continuing to use) chemical fertilizers. For all farmers in the region, that's true.