Answer: Option (a) is the correct answer.
Explanation:
Substance which show pH less than 7 are acidic in nature. For example, HCl being a strong acid has pH very less than 7.
Substances which show pH greater than 7 are basic in nature. For example, NaOH being basic in nature has pH greater than 7.
Whereas substances which have pH equal to 7 are neutral in nature, that is, they are neither acidic nor basic.
For example, water is neutral in nature.
Thus, we can conclude that the statement it contains only water is true about an unknown sample with a pH of 7.