Answer: Option (c) is the correct answer.
Step-by-step explanation:
When a substance is acidic in nature then it turns blue litmus into red. Whereas when a substance is basic in nature then it turns red litmus into blue litmus.
Therefore, we can conclude that a litmus paper helps in determining the pH of a solution because if the pH is less than 7 then the substance is acidic in nature.
On the other hand, when pH is more than 7 then the substance is basic in nature.
Hence, litmus paper can be used to determine the relative pH of a substance.