Using the pKa values of each compounds conjugate acid, predict the stronger base

Question

Using the pKa values of each compounds conjugate acid, predict the stronger base for the compounds listed:
A) H- B)OH- C)NH3

So working problems like this, since it says to use the pKa values of the conjugate acids, does that mean NH3's pKa would be roughly 9 because its conjugate acid is a protonated amine? Making the answer A? I'm trying to understand this so I can apply it to other questions similar to this, so please break this down as simply as possibly for me, or any other helpful tips would be greatly appreciated.

Explanation / Answer

NH4+ <=========> NH3 + H+ ; pKa = 9.25

H2O <=========> OH- + H+ ; pKa = 15.7

H2 <=========> H- + H+ ; pKa = 23.0

As we know that, pKa is inversily proportional to Acidic strength. In other words, pKa is directly proportional to basicity of conjugate base.

From the above values, H- is the strongest base among H-, OH-, NH3.

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