To determine which solution will change red litmus paper to blue, we need to identify which of the given solutions is basic in nature. A solution that is basic will have a pH greater than 7 and will cause red litmus paper to turn blue.
Among the options provided:
- a) HCl (aq) – This is hydrochloric acid, which is acidic and will not change red litmus to blue.
- b) NaCl (aq) – Sodium chloride is a neutral salt formed from a strong acid and a strong base. It will not change the color of litmus paper.
- c) NaOH (aq) – Sodium hydroxide is a strong base. It has a pH greater than 7 and will turn red litmus paper blue.
- d) CH₃OH (aq) – This is methanol, which is neither acidic nor basic in its aqueous form. It also will not change the color of litmus paper.
Therefore, the correct answer is c) NaOH (aq) as it is the only solution that will change red litmus to blue.