There was a little exercise where you have to put a word in the gap, the sentence was like this:
"In a recent film, during ____ a family was waiting to have a meal..."
Answer was "which", but I put "when", then I searched for this topic and on some forum, it was written that when and which both are grammatically correct, but "when" is redundant, I agree in this case, because "during" also indicates time, but I want to know if "during when" phrase has any grammatical issue?