Increasing pKA=decreasing acidity. this tests the inductive effect.
The more electronegative the atom=the more it makes the overall compound want to donate a proton=strong acid. So the compound with the F will be most acidic. O is second most electronegative so 2 and Br is less electronegative than O. So F>O>B and least acidic C.
Applying this to HF, HBr, H2O, HI, H2S we would instead do:
Comparing down a group, the bigger the atom, the stronger the acid. (HI, HBr, HF)
Comparing across a period (HF and H2O), the more electronegative, the more acidic, correct?