The higher the hydronium ion concentration of a solution, the its ph.

a. lower
b. higher

The higher the hydronium ion concentration of a solution, the lower its pH.

Explanation:

Consider a high hydronium concentration, 1 mol·L⁻¹.

pH = -log[H₃O⁺] = -log1 = 0

Now consider a low hydronium ion concentration, 10⁻¹⁴ mol·L⁻¹.

pH = -log(10⁻¹⁴) = 14

Thus, a high hydronium ion concentration means a low pH.

the answer is na (sodium)