The best choice is hypochlorous acid (HClO) because it has the nearest value of pK to the desired pH.
pKa of hypochlorous acid is 7.54
If we know pKa and pH values, we can calculate the required ratio of conjugate base (ClO⁻) to acid (HClO) from the following equation:
pH=pKa + log(conc. of base)/( conc. of acid)
7.66=7.54 + log(conc. of base)/( conc. of acid)
7.66 - 7.54 = log(conc. of base)/( conc. of acid)
0.12 = log(conc. of base)/( conc. of acid)
(conc. of base)/(conc. of acid) = 10⁻⁰¹² = 0.76
