Which word could be used to describe something that is not harmful at all?

The word "innocuous" is used to describe something that is harmless or not likely to cause any injury or adverse effects. It conveys the idea that an object, action, or even a statement poses no threat to well-being or safety. This word is often used in contexts where the intention or effect is benign, suggesting that something can be safely tolerated or is inoffensive.

In contrast, the other options carry negative connotations. "Nefarious" refers to something that is wicked or villainous, typically involving immoral or criminal acts. "Insidious" implies a gradual but harmful effect, often subtly or stealthily leading to negative consequences. "Malicious" indicates a desire to cause harm or pain to others, highlighting intentionality in the harm caused. Therefore, "innocuous" stands out as the appropriate choice for describing something completely safe and non-threatening.