Hello. I will be very grateful for your advice.

Which is correct:
a) He had a new job which he was very proud of.
b) He had a new job that he was very proud of.
c) He had a new job he was very proud of.
(I, for some reason, prefer a)
Thank you very much for help.

All are correct, as long as you don't mind a preposition at the end of the sentence!! (Used to be a big no-no!)

It is a matter of preference, but all are grammatically correct.