Acetic acid solution turns:

<b>Correct Option (A):</b> Acetic acid ($\text{CH}_3\text{COOH}$) is an organic acid. In an aqueous solution, it dissociates to release hydrogen ions ($\text{H}^+$). These free hydrogen ions are responsible for its acidic nature, which turns blue litmus paper red.<br><br><b>Incorrect Option (B):</b> Red litmus paper turns blue only in the presence of basic (alkaline) substances, whereas acetic acid is acidic.<br><b>Incorrect Options (C) & (D):</b> Litmus paper does not become colourless; it acts as an indicator by changing colour between red and blue depending on the pH of the medium.