Must the entirety of Ramadan be made up for, or only the day on which masturbation occurred, taking into consideration not remembering whether the fast was completed thereafter or not?

Masturbation is forbidden, and it is more severely forbidden in Ramadan. Repentance from it is obligatory. If a fasting person masturbates during the daytime in Ramadan and semen is discharged, their fast is invalidated, and they must make up that day. Expiation (kaffara) is not obligatory, according to the more preponderant opinion. If one doubts the fasting of the rest of the month, they must make up the days they doubted their fasting, because the default assumption is not having fasted. However, mere doubt about the validity of the fast after its completion does not invalidate the fast.