Here the word pure water means that there is nothing present in water only and only 100 % water :) .
As we know that water can dissociate producing H⁺ (H₃O⁺) and OH⁻ as shown below,
H₂O ↔ H⁺ + OH⁻
or,
H₂O + H₂O ↔ H₃O⁺ + OH⁻
Now,
The amount of H⁺ and OH⁻ are very small in pure water and their concentration is as,
[H⁺] = 1 × 10⁻⁷ M (Option C is correct)
[OH⁻] = 1 × 10⁻⁷ M,
Taking antilog of [H⁺],
-log [H⁺] = pH
-log [1 × 10⁻⁷] = pH
pH = 7 (Option A is correct)
But,
Option B (i.e. 1.0 × 10⁷ M) concentration is incorrect because it is very high concentration both for H⁺ ions ( making it acidic) and for OH⁻ ions (making it basic).
