This answer is not correct. I think that you thought that somehow the carbonyl oxygen atom was protonated prior to the attack by ammonia. There is no Brønsted acid in this reaction mixture, nor is one needed. The carbonyl carbon atom is very electropositive already and does not need to be activated by an acid.
Alternatively you may have been thinking of the second step in the mechanism where a proton is transferred to the oxide ion. This proton comes from the -NH3+group so it changes to -NH2 as shown in answer "d".