Я так новичок в Django, поэтому BaseInlineFormУстановите, как работать, я не понимаю.
class BaseAnswerInlineFormSet(forms.BaseInlineFormSet):
def clean(self):
super().clean()
has_one_correct_answer = False
for form in self.forms:
if not form.cleaned_data.get('DELETE', False):
if form.cleaned_data.get('is_correct', False):
has_one_correct_answer = True
break
if not has_one_correct_answer:
raise ValidationError('Mark at least one answer as correct.', code='no_correct_answer')