Which of the following has the highest _pK_a value?

Iodoacetic acid

Higher the electronegativity of a halogen, greater will be its electron withdrawing inductive effect (-I effect), and stronger will be the acid. Since electronegativity of halogens decrease in the order F > Cl > Br > I, iodoacetic acid will be the weakest acid and hence will have the highest _pK_a value.