I answered "Czy przygotowałes obiad?".
It is correct but for me it sounds weird. We use "zrobić" more often. And remember you can skip "czy".
Hmmm. Multiple choice requires both "Zrobiłaś obiad?" and "Zrobiłeś kolację?" ?. Yet, elsewhere, "obiad" seems to be "lunch", and "kolacja" seems to be "dinner".
why not, "czy zjadłeś kolację?" ?
Because the sentence mentions making, not eating. Your sentence is perfectly correct... it just doesn't mean what it was supposed to mean ;)