What is the pH of a solution with a hydronium ion concentration of
7.00 X 10-7 mol/L?

a. 1.21 b. 7.00
c. 6.35 d. 6.15
e. none of the above.

The definition of pH is:

pH = -log([H^+])

You are given that

[H^+] = 7.00 \times 10^{-7}

Take the common log of that and calculate its negative.