My father filed a petition for me in 1999 when I was 20 years old and he was an LPR. He naturalized in 2003. My visa became available this year but then I aged out. I was told that I could opt out from F1 and apply for an F-2B retention. Will I be approved of that?

Yes.

how sure are you of that that I would be approved of an F-2B retention.?

very sure.