Think about what gametes the father could have given the child. If the mother contributed an "A", then the father gave two "C"s, if the mother contributed a "C" then the father contributed an "A" and a "C".
In meiosis I, the father's chromosomes duplicate, so he has AA and CC - normally, at the end of meiosis I, these would be pulled into separate cells, so one of the resulting cells would be AA (and then in meiosis II, split into two further cells both with one "A") and CC (which in meiosis II splits into two further cells both with one "C"). The final result would be four gametes (sperm cells), two with "A"s and two with "C"s.
Since this is tagged "homework" I'm not going to give the final answer - but think about what phase of meiosis would have to go wrong to create an "AC" sperm, and what phase of meiosis would have to go wrong to create a "CC" sperm.