Which acid is the best choice to create a buffer with ph= 7.66?

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

Yes, endothermic means it turns cold. and some chemical changes include change of tempature.