Which definition best describes "pernicious"?

The term "pernicious" is defined as having a harmful effect, particularly in a subtle or insidious way. It conveys the idea that something is not just harmful, but that it can lead to deadly consequences. This aligns perfectly with the choice indicating that the term means "extremely harmful and deadly," which captures the intensity and seriousness of the term.

The other options do not reflect the meaning of "pernicious." For instance, the idea of being "harmless and trivial" directly contradicts the definition, as it suggests a lack of impact or danger. Similarly, describing something as "extremely beneficial" or "helpful in nature" undermine the concept of harm that is inherent in "pernicious." Therefore, the choice that highlights its extreme harmfulness and potential lethality is the most accurate representation of the term.