Which of the following is the opposite of "insidious"?

The term "insidious" refers to something that operates in a gradual, subtle way but with harmful effects. It often implies a sense of deceit or stealthiness, where negative consequences develop over time without immediate detection. The opposite of this concept would be something that is safe or harmless, which means that it does not pose a threat or risk.

Choosing "harmless" makes sense because it denotes a lack of danger or malice, contrasting directly with the underlying negative implications of "insidious."

The other options, while they may share some characteristics with "insidious," do not provide a clear opposite. "Subtle" suggests a fine or delicate quality which aligns with the nature of "insidious." "Deceptive" has a similar connotation to "insidious," as both involve misleading appearances or intentions. "Gradual" also parallels the idea of something that develops over time, which is characteristic of an insidious nature but does not represent its negation. Therefore, "harmless" stands out as the correct answer as it directly counters the harmful and deceitful connotations associated with the word "insidious."