Explanation:

- Disappear into thin air is an idiom used to describe something or someone vanishing without a trace.

- Option 1: Lost forever

- This suggests something is gone and cannot be found again.

- Doesn't capture the suddenness implied by the idiom.

- Option 2: Become forgetful

- Relates to memory issues, not disappearing.

- Not relevant to the idiom's meaning.

- Option 3: Become uncaring

- Represents a lack of concern or empathy.

- Unrelated to the concept of something vanishing.

- Option 4: Disappear suddenly

- Correct Answer

- Captures the essence of the idiom, emphasizing a quick disappearance without explanation.