If the German sentence used "Die Rest
e", then "leftovers" would be a correct option. As it is, we're not talking about "leftovers"--food remaining after a meal; excess food eaten later--but rather some additional, uncountable thing (perhaps food, perhaps drink). Note the use of the singular conjugation, ist. Not "leftover
s are" but "remainder
I think "leftovers" is better translated as Essensreste oder Speisereste. See dict.cc. Possibly "Rest
The German sentence we are asked to translate uses "Rest" (singular), which simply means "remainder, remnant, rest", which could refer (as AllenJudge3 remarks) to people, food, decorations, etc.
When I read the sentence, I do not think of "leftovers", which is the food from a previous meal, but rather additional food (chairs, people, service items, etc) that have not been brought out to the current location. E.g.,
Gast: Es gibt sehr viele Leute hier an deinen Party, aber nicht viele Essen. Du habst keine genuge Essen.
Gastgeber: Ich habe ausreichend. Der Rest is in der Küche.
Previously Duo had "Der rest ist einfach Gluck", with rest here meaning what remains. or "the remainder". I suppose it also means "the Leftovers" as I read through this, But what came to my mind is The rest (of the people, or visitors) are in the Kitchen. "The Rest are in the Kitchen" is what I had but it is marked wrong. Would a native German speaker every say The Rest are in the Kitchen by saying "Der Rest ist in der Kuche?