"What vegetable is the healthiest?"
So all of the sudden, 什么 can go at the FRONT of the sentence!? Why can't it be "the healthiest vegetable is what?" (最健康蔬菜是什么?)
Your way is viable too, only you need to put 的 before 蔬菜。
什么 works approximately like What, both as an interrogative pronoun and as a determinant. It is even more flexible, as you can put it at the end as well as in front of a sentence.
Thank you. That makes sense. I was forgetting that "healthy" has to have 的 after it. That's good to know that "what" can go at the start or end of a sentence.
"最健康的蔬菜是什么?" Still marked wrong as of 2019-03-16. Reported.
Asked native speaker, they said this should be acceptable as it means the same as "什么蔬菜最健康," they just emphasized (like Keith_APP above) that you need the 的 if you're going to switch the order around like this.
Could you also say 哪個蔬菜 in this context? If not, what is the difference?
You cannot say 个 because it does not apply to all types of vegetable. You can say 哪种蔬菜/Which type of vegetable.
Leafy vegetables are counted by 棵, such as salad, lettuce, cabbage.
Round, oval vegetables such as pumpkin, potato, onion are counted by 个, small ones (e.g. cherry tomatoes, radish) can be counted in 颗 .
Relatively long vegetables are counted in 根 , such as carrot, a branch of celery, etc.
哪種蔬菜 is "what (kind of) vegetable," used when the options are given. We often mix use them in many cases, though.
The viable ways are:
什么蔬菜 最健康？ （subject + adjective）
什么蔬菜 是 最健康的？ （subject + Be + nominalized adjective）