Answer:
pH = 3 (though the option is missing)
Step-by-step explanation:
We have a solution with a concentration of the hydrogen ion, [H⁺] = 0.001 mol/L = 0.001 M. We can calculate the pH using its definition.
pH = -log [H⁺]
pH = -log 0.001
pH = 3
Since the pH < 7, the solution is acid. Other way to recognize that a solution is acid is when [H⁺] > 10⁻⁷ M at 25°C.