Correct?

Which version is grammatically correct and natural to native speakers:

     (a) A friend of mine commented the sentence above as follows:
     (b) A friend of mine commented on the sentence above as follows:
     (c) A friend of mine commented (on) the above-mentioned sentence as follows:
     (d) A friend of mine commented (on) the sentence above in the following:

   Thanks..

Only b and c is you include the ‘on’ (it is not possible to omit it).