Nitrogen is crucial to the marine life and it is disappearing because it cannot be assimilated by most organisms in the water.
Option a is a solution acidic ([OH⁻] = 7.0x10⁻⁹ M).
To know if a solution is acidic we need to calculate the pH and it must be lower than 7. A value of pH equal to 7 is a neutral solution and a solution with a pH value higher than 7 is a basic solution.
a. For the [OH⁻] = 7.0x10⁻⁹ M we have:
Now, the pH is:
This solution is acidic (pH < 7)
b. [H₃O⁺] = 8.5x10⁻⁸ M
This is not an acidic solution. Is a neutral one (pH around to 7).
c. [OH⁻] = 2.5x10⁻⁶ M
Then, the pH is:
Hence, this is not an acidic solution. It is basic (pH > 7).
Therefore, option a is a solution acidic ([OH⁻] = 7.0x10⁻⁹ M).
I hope it helps you!