Step 1: Understanding the word "innocuous."
"Innocuous" means harmless or not likely to offend or cause harm. The antonym should describe something that causes harm or is dangerous.
Step 2: Analyzing the options.
- Inoffensive means something that does not cause offense, which is similar in meaning to innocuous, so it is not the correct antonym.
- Harmful means something that causes harm or damage, which is the correct antonym of innocuous.
- Gentle means kind or soft, which does not necessarily oppose innocuous.
- Insightful means having deep understanding, unrelated to the meaning of innocuous.
Step 3: Conclusion.
"Harmful" is the correct antonym because it means causing harm, which is the opposite of harmless or innocuous.
Thus, the correct answer is 2. Harmful.
Final Answer:
\[
\boxed{\text{The correct answer is 2. Harmful.}}
\]