Which word best completes the sentence?

"Obscure" suggests something that is not clear or is difficult to understand. While the adaptations of poison frogs may be complex, referring to them as "obscure" does not reflect the active and robust nature of their evolutionary changes. This choice fails to capture the essence of the frogs' successful adaptation to their environment.

"Ominous" implies a sense of foreboding or threatening. While the neurotoxins themselves can be dangerous, this word does not relate to the frogs' own adaptations and resilience. It misrepresents the positive aspect of their evolutionary success in dealing with toxins, making it an unsuitable choice.

"Vigorous" conveys strength, energy, and robustness, which aligns perfectly with the notion of the frogs adapting successfully to the presence of a potent neurotoxin. This term effectively captures the essence of their evolutionary changes, highlighting the strength of their adaptations to avoid self-toxicity.