You put what you are the day you file. You will be single, and use your mom on FAFSA. You don’t put a fiance; you may break up with him the next day and never marry.
If this is USC in Calif, then you’ll also be filling out CSS Profile. And I think USC also requires the financial info of your non-custodial parent. Is your dad alive? Edit: I now see that your dad is deceased. Very sorry about that.