“Some” is the real killer here. The stimulus accepts there might be a few predators that are immune to monarch poison and therefore not deterred by the viceroy’s appearance. There are many other predators that aren’t immune, though, and that’s plenty.

The causal mechanism at play in the stimulus involves viceroys being protected because of their resemblance to monarchs, not because they themselves are toxic. On the stimulus’ proposed explanation, individual monarchs are protected by their toxicity because most monarchs are toxic, and individual viceroys are protected because they look like monarchs.

This strengthens the stimulus’ proposed causal mechanism by establishing a connection between the two species – they don’t, for example, live on different continents.

This provides an alternate explanation for why the viceroys are seldom preyed upon. It’s not because they look like monarchs, it’s because they themselves are poisonous!

(D) is perhaps tricky because the idea that viceroys don’t eat milkweed might suggest they aren’t poisonous. But all it really means is they aren’t milkweed poisonous. They could still be poisonous in other ways.

Even if no other species does so, monarch butterflies do use their toxicity to avoid predators. That’s enough to support the stimulus’ proposed explanation that looking like monarchs helps viceroys avoid predators.