The caterpillar of the monarch butterfly feeds on milkweed plants, who

The conclusion states that the viceroy species are not toxic and have some disguised protection.
We need to show that this type of butterfly is toxic too.
Hence D

The caterpillar of the monarch butterfly feeds on milkweed plants, whose toxins make the adult monarch poisonous to many predators. The viceroy butterfly, whose caterpillars do not feed on milkweed plants, is very similar in appearance to the monarch. Therefore, it can be considered that the viceroy is so seldom preyed on because of its visual resemblance to the monarch.

Which of the following, if it were discovered to be true, would most seriously undermine the argument?

(A) Some predators do not have a toxic reaction to insects that feed on milkweed plants. - WRONG. Exceptions are possible, does not matter then.

(B) Being toxic to predators will not protect individual butterflies unless most members of the species to which such butterflies belong are similarly toxic. - WRONG. Not certain if its about monarch or viceroy.

(C) Some of the predators of the monarch butterfly also prey on viceroys. - WRONG. Then what..!! There can still be chances where extreme opposite possibilities are possibility.

(D) The viceroy butterfly is toxic to most predators. - CORRECT. Another reason that viceroys are seldom preyed, not only because of its appearance similar to monarch. This provides an alternative to visual resemblance, thus weakening the conclusion.

(E) Toxic to predators is the principal means of protection for only a few butterfly species. - WRONG. Which ones???

Answer D.

Conclusion is Viceroy is seldom preyed. Evidence in support of the conclusion is they are similar in appearance to monarch.

We have to weaken the conclusion. So that means we have to prove that viceroy is not seldom preyed. But option D would fortify the conclusion.

Please share your insight.

Monarchs feed on milkweed and so are poisonous to predators.
Viceroys do not feed on milkweed but look like monarchs.
That is why viceroys are seldom preyed upon.

The conclusion is that the reason viceroys are seldom preyed upon is that they look like monarchs.
The conclusion is NOT that the viceroys are seldom preyed upon. That is given as a fact. It is observable. We are concluding on the why, the hypothesis that will explain why they are seldom preyed upon. That is where the author is giving her opinion.

We need to undermine the hypothesis that because viceroys look like monarchs, they are not preyed upon. (The predators think that they are the poisonous monarchs)

(A) Some predators do not have a toxic reaction to insects that feed on milkweed plants.

Irrelevant. Some predators may not have a toxic reaction to milkweed but many do (as given).

(B) Being toxic to predators will not protect individual butterflies unless most members of the species to which such butterflies belong are similarly toxic.

We know that all monarchs are toxic. Do monarchs and viceroys belong to same species? We don't know. There isn't much to evaluate here.

(C) Some of the predators of the monarch butterfly also prey on viceroys.

It doesn't say anything about why predators who do not prey on monarchs do not prey viceroys too. That is what we are interested in finding out. The few predators who prey on monarch are out of scope for us.

(D) The viceroy butterfly is toxic to most predators.

This could explain why viceroys have few predators too. They are toxic to predators too. Then, it may have nothing to do with the fact that they look like monarchs. They may not feed on milkweed but they are still toxic to predators because of some other chemicals they have.
Hence, this weakens our hypothesis that predators don't feed on viceroys because viceroys look like monarchs.

(E) Toxic to predators is the principal means of protection for only a few butterfly species.

Irrelevant.

Answer (D)