Which of the following compounds will turn a piece of red litmus paper to a bluish color? 

• A. HI
• B. C₆H₅NH₂
• C. CH₃COOH
• D. HOCN

Answer: (B)

Verified step by step guidance

1. Understand that red litmus paper turns blue in the presence of a basic (alkaline) substance and remains red or turns red in acidic or neutral substances.
2. Identify the nature of each compound: HI is a strong acid, CH3COOH (acetic acid) is a weak acid, HOCN (cyanic acid) is also acidic, and C6H5NH2 (aniline) is a weak base due to the lone pair on the nitrogen atom.
3. Recall that acids release H+ ions in solution, which will keep red litmus paper red, while bases accept H+ ions or release OH- ions, turning red litmus paper blue.
4. Conclude that among the given compounds, only C6H5NH2 is basic enough to turn red litmus paper blue because it can accept protons, indicating basic behavior.
5. Therefore, the compound that will turn red litmus paper blue is C6H5NH2 (aniline).