How will the right answer fit in terms of support and conclusion?

Only the right answer will support disagreeing with the conclusion.

Highlight the main conclusion in the passage, if there is one:

Animal feed should not include genetically modified plants.

[CONCLUSION]. [SUPPORT].

The conclusion changes the subject big time. The support is only about “potatoes” but the conclusion switches that to “plants”. The right answer will call out why that doesn’t work.

Map the wording of the answers to the wording of the passage:

(A) Potatoes are not normally a part of the diet of laboratory rats.

Boom! This is calling out why support about potatoes doesn’t establish a conclusion about all “genetically modified plants”. The “weakened immune system” the potato-fed rats developed could be caused by a potato-specific reason. That would blow this conclusion up for sure.

(B) The rats tended to eat more…at the beginning of the 30 days than…

The rats ate less of the potatoes as time went on. If you’re connecting that to recommendation in the conclusion, you must be adding in at least a couple steps of your own reasoning. The passage never compares “the beginning of the 30 days” to the end, so this is bringing in a whole new factor that isn’t clearly relevant.

(C) Intestinal deformities at birth…

Stop. The author says the rats “develop” the deformities, which they claim is from eating GMOs. Rats being born with deformities has nothing to do with genetically modified food causing deformities later.

(D) …have the same nutritional value…

The passage doesn’t actually mention nutrition, so you’re adding in your own assumption if you think this weakens the conclusion. The author’s support is about “deformities”, not malnourishment. This info doesn’t impact the argument.

(E) The researchers conducting the study…

…are never mentioned. It would have to be obvious how info about them affects the conclusion. But the passage never tries to “explain” the deformities. This info doesn’t support disagreeing with the conclusion.

(A) is the correct answer.