The rule is more complicated. You drop the definite article only when you are using the singular. http://italian.about.com/library/fare/blfare124a.htm
That's the second time 'my parents are eating' has been marked wrong, when 'my parents eat' is given as a correct answer - why? Surely they're both present tense?
It seems like "a" can be everything at/in/the/on, i really don't understand it...can anyone explain how to use this "a" and when does it mean which of its meanings?
Mangiare means to eat. Lunch is too specific, they may be referring to breakfast (you don't really know here). If you wanted to say lunch you would use "pranzo"
Mezzogiorno is noon but not lunchtime? I would have thought both English words would be accepted since the sentence is about eating.