Hi, there. I'm happy to help with this.

In the prompt:
Sentence #1 frames the situation and states the problem that needs to be resolved --- a vitamin-A deficiency
Sentence #2 presents a plan designed to solve the problem. Th
Sentence #3 present some support for the plan.

The plan's success depends on assumptions, such as:
a) the SPK004 sweet potato will grow successfully there
b) the folks there will be willing to plant & eat the SPK004 sweet potato instead of their traditional sweet potatoes.
c) nutritionally, nothing else would be lost in a switch from traditional sweet potatoes to the SPK004 sweet potato

OK, so we want answer that will strengthen the argument. Often on the GMAT CR, if the question is asking you to strengthen the argument, two or three of the incorrect answers will be weakeners. Conversely, when the CR asks you to weaken the argument, two or three of the incorrect answers will strengthen the argument. Always be on the lookout for incorrect answers that do the opposite of what you were asked to do. Let's look at these answer choices.

A. There are other vegetables currently grown in the region that contain more beta-carotene than the currently cultivated varieties of sweet potato do.
Well, if there are other vegetables currently grown that have more beta-carotene, then it would seem the folks there are already getting enough beta-carotene, and for some reason, they are not properly converting it to vitamin-A. It's unclear what the problem would be, but certainly adding a new sweet potato with more beta-carotene wouldn't necessarily solve anything. The plan will not solve the problem. This answer is a weakener, so it's incorrect.

B. The flesh of SPK004 differs from that of the currently cultivated sweet potatoes in colors and textures, so traditional foods would look somewhat different when prepared from SPK004.
When food is different looking --- especially different in both color and texture --- it can be hard to get folks set in their ways to try it. If a large proportion of the folks in these places either decide not to try the new sweet potato or try it and decide they don't like it, then many people will not eat it and will not get the benefit of the beta-carotene, and thus the vitamin-A deficiency will continue. The plan will not solve the problem. This answer is a weakener, so it's incorrect.

C. For successful cultivation of SPK004, a soil significantly richer in nitrogen is needed than is needed for the varieties of sweet potato currently cultivated in the region.
Hmmm. If the SPK004 sweet potato needs soil much richer in nitrogen, it sounds as if it's at least possible that it won't grow particularly well in these environments. If the farmers there can't grow it well or at all, then it won't replace the traditional sweet potatoes, and no one will benefit. The plan will not solve the problem. This answer is a weakener, so it's incorrect.

D. There are no other varieties of sweet potato that are significantly richer in beta-carotene than SPK004 is.
This is a particularly interesting one. We already can infer that the SPK004 sweet potato has more beta-carotene than the the traditional sweet potatoes in this region. That's the comparison that is meaningful in this argument. This answer choice provides the added information --- the SPK004 sweet potato also contains more beta-carotene than any other variety of sweet potato anywhere else --- China, Uganda, New Zealand, Alabama, etc. Well, that's interesting, but it's not particularly relevant. We already know the important comparison ---- the SPK004 sweet potato vs. sweet potatoes traditionally grown in these parts of South America. That's the part that is pertinent to the argument. The fact that the SPK004 sweet potato also contains more beta-carotene than any other variety of sweet potato in other places --- strictly speaking, this is irrelevant to the argument. Therefore, it is not a strengthener. This answer is incorrect.

E. The currently cultivated varieties of sweet potato contain no important nutrients that SPK004 lacks.
Well, we have just nuked the first four answers, so we hope this one works! Fortunately, this is almost identical to assumption (c) above. So, in switching from the traditional sweet potatoes to the SPK004 sweet potato, these folks will gain beta-carotene and will not lose anything other nutrient. This is a strengthener --- in fact, it's one of the assumptions on which the plan is based.
If we were to deny this and assert the opposite of (E) --- in switching from the traditional sweet potatoes to the SPK004 sweet potato, these folks would beta-carotene but lose some other important nutrient --- then, that would weaken the argument, because it seems like the plan would just replace one problem (a vitamin-A deficiency) with another problem (a deficiency of some other nutrient). Denying (E) weakens the argument, which is further evidence that it's a strengthener.
For all these reasons, (E) is the best answer.

Notice that one strategy that helped significantly in the analysis of this question was finding assumptions before we approached the answer choices. That won't always work, but it works enough that it's a useful strategy. See this post for more on that strategy.

Does all this make sense? Please let me know if you have any further questions.

Mike
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Mike McGarry
Magoosh Test Prep

Although its not an assumption question , its a strengthen question and the technique mentioned in Powerscore CR bible is for assumption questions only, I am wondering how come the technique works here
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