Translation:Don't eat unhealthy food.
Both unhealthy food and unhealthy foods should work. There is nothing specifying that it cant be plural.
Usually in English food is uncountable. So,unhealthy foods doesn't sound natural.
It may not sound natural but you can pluralize 'food'.
"Various types of food(s) were eaten..."
Shouldn't "Don't eat something unhealthy" be considered as a correct answer as well?
The phrase doesn't translate to "food" though. "Dongxi" is "thing." I just watched a Shirokuma Cafe (a Japanese anime) episode where Panda ate a kebab stick (along with the kebab). This sentence would be most appropriate in that context.
Actually, the stick was probably made of bamboo and bamboo is part of a healthy diet for pandas. I am joking, as was the episode in question.