Explanation:

- Inadvertent: This word means something that was not intended or planned; an action done without deliberate thought.

- Option 1: Insignificant: This means something that is small or not important. Although it might seem similar, it doesn't convey the lack of intention.

- Option 2: Careless: This refers to doing something without sufficient attention or thought. It's related but implies negligence rather than a lack of intention.

- Option 3: Unintentional: This captures the meaning of inadvertent perfectly. It means something done without intention.

- Option 4: Difficult: It means something hard to do or understand. This is not related to the concept of being inadvertent.