Explanation:

- Blue litmus paper is used to test if a solution is acidic or basic.

- When in contact with an acidic solution, blue litmus paper changes color.

- ? Option 2: Red

- Blue litmus turns red in acidic conditions.

- This is the correct behavior when exposed to acids.

- Option 1: Green

- Not applicable to blue litmus paper.

- Could occur under different conditions unrelated to acidity.

- Option 3: Yellow

- This change isn’t typical of standard blue litmus reactions.

- Option 4: Brown

- Brown is not a color associated with litmus paper reactions.